refactor(grid): move out grid_* functions from screen.c

Also normalize some types. use "size_t" for unsigned array offsets.
Fix -Wconversion issues missed as screen.c is missing this check.

10 files changed, 777 insertions, 752 deletions

diff --git a/src/nvim/ex_getln.c b/src/nvim/ex_getln.c
index 1932c02cf2..d4cea15d20 100644
--- a/src/nvim/ex_getln.c
+++ b/src/nvim/ex_getln.c
@@ -3664,7 +3664,7 @@ static void cursorcmd(void)
 static void cmd_cursor_goto(int row, int col)
 {
   ScreenGrid *grid = &msg_grid_adj;
-  screen_adjust_grid(&grid, &row, &col);
+  grid_adjust(&grid, &row, &col);
   ui_grid_cursor_goto(grid->handle, row, col);
 }
 
diff --git a/src/nvim/grid.c b/src/nvim/grid.c
new file mode 100644
index 0000000000..b150d3819e
--- /dev/null
+++ b/src/nvim/grid.c
@@ -0,0 +1,708 @@
+// This is an open source non-commercial project. Dear PVS-Studio, please check
+// it. PVS-Studio Static Code Analyzer for C, C++ and C#: http://www.viva64.com
+
+#include "nvim/arabic.h"
+#include "nvim/highlight.h"
+#include "nvim/vim.h"
+#include "nvim/grid.h"
+#include "nvim/ui.h"
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "grid.c.generated.h"
+#endif
+
+// temporary buffer for rendering a single screenline, so it can be
+// compared with previous contents to calculate smallest delta.
+// Per-cell attributes
+static size_t linebuf_size = 0;
+
+
+/// Determine if dedicated window grid should be used or the default_grid
+///
+/// If UI did not request multigrid support, draw all windows on the
+/// default_grid.
+///
+/// NB: this function can only been used with window grids in a context where
+/// win_grid_alloc already has been called!
+///
+/// If the default_grid is used, adjust window relative positions to global
+/// screen positions.
+void grid_adjust(ScreenGrid **grid, int *row_off, int *col_off)
+{
+  if ((*grid)->target) {
+    *row_off += (*grid)->row_offset;
+    *col_off += (*grid)->col_offset;
+    *grid = (*grid)->target;
+  }
+}
+
+/// Put a unicode char, and up to MAX_MCO composing chars, in a screen cell.
+int schar_from_cc(char_u *p, int c, int u8cc[MAX_MCO])
+{
+  int len = utf_char2bytes(c, p);
+  for (int i = 0; i < MAX_MCO; i++) {
+    if (u8cc[i] == 0) {
+      break;
+    }
+    len += utf_char2bytes(u8cc[i], p + len);
+  }
+  p[len] = 0;
+  return len;
+}
+
+/// clear a line in the grid starting at "off" until "width" characters
+/// are cleared.
+void grid_clear_line(ScreenGrid *grid, size_t off, int width, bool valid)
+{
+  for (int col = 0; col < width; col++) {
+    schar_from_ascii(grid->chars[off + (size_t)col], ' ');
+  }
+  int fill = valid ? 0 : -1;
+  (void)memset(grid->attrs + off, fill, (size_t)width * sizeof(sattr_T));
+}
+
+void grid_invalidate(ScreenGrid *grid)
+{
+  (void)memset(grid->attrs, -1, sizeof(sattr_T) * (size_t)(grid->Rows * grid->Columns));
+}
+
+bool grid_invalid_row(ScreenGrid *grid, int row)
+{
+  return grid->attrs[grid->line_offset[row]] < 0;
+}
+
+static int line_off2cells(schar_T *line, size_t off, size_t max_off)
+{
+  return (off + 1 < max_off && line[off + 1][0] == 0) ? 2 : 1;
+}
+
+/// Return number of display cells for char at grid->chars[off].
+/// We make sure that the offset used is less than "max_off".
+static int grid_off2cells(ScreenGrid *grid, size_t off, size_t max_off)
+{
+  return line_off2cells(grid->chars, off, max_off);
+}
+
+/// Return true if the character at "row"/"col" on the screen is the left side
+/// of a double-width character.
+///
+/// Caller must make sure "row" and "col" are not invalid!
+bool grid_lefthalve(ScreenGrid *grid, int row, int col)
+{
+  grid_adjust(&grid, &row, &col);
+
+  return grid_off2cells(grid, grid->line_offset[row] + (size_t)col,
+                        grid->line_offset[row] + (size_t)grid->Columns) > 1;
+}
+
+/// Correct a position on the screen, if it's the right half of a double-wide
+/// char move it to the left half.  Returns the corrected column.
+int grid_fix_col(ScreenGrid *grid, int col, int row)
+{
+  int coloff = 0;
+  grid_adjust(&grid, &row, &coloff);
+
+  col += coloff;
+  if (grid->chars != NULL && col > 0
+      && grid->chars[grid->line_offset[row] + (size_t)col][0] == 0) {
+    return col - 1 - coloff;
+  }
+  return col - coloff;
+}
+
+/// output a single character directly to the grid
+void grid_putchar(ScreenGrid *grid, int c, int row, int col, int attr)
+{
+  char_u buf[MB_MAXBYTES + 1];
+
+  buf[utf_char2bytes(c, buf)] = NUL;
+  grid_puts(grid, buf, row, col, attr);
+}
+
+/// get a single character directly from grid.chars into "bytes[]".
+/// Also return its attribute in *attrp;
+void grid_getbytes(ScreenGrid *grid, int row, int col, char_u *bytes, int *attrp)
+{
+  size_t off;
+
+  grid_adjust(&grid, &row, &col);
+
+  // safety check
+  if (grid->chars != NULL && row < grid->Rows && col < grid->Columns) {
+    off = grid->line_offset[row] + (size_t)col;
+    *attrp = grid->attrs[off];
+    schar_copy(bytes, grid->chars[off]);
+  }
+}
+
+
+/// put string '*text' on the window grid at position 'row' and 'col', with
+/// attributes 'attr', and update chars[] and attrs[].
+/// Note: only outputs within one row, message is truncated at grid boundary!
+/// Note: if grid, row and/or col is invalid, nothing is done.
+void grid_puts(ScreenGrid *grid, char_u *text, int row, int col, int attr)
+{
+  grid_puts_len(grid, text, -1, row, col, attr);
+}
+
+static ScreenGrid *put_dirty_grid = NULL;
+static int put_dirty_row = -1;
+static int put_dirty_first = INT_MAX;
+static int put_dirty_last = 0;
+
+/// Start a group of grid_puts_len calls that builds a single grid line.
+///
+/// Must be matched with a grid_puts_line_flush call before moving to
+/// another line.
+void grid_puts_line_start(ScreenGrid *grid, int row)
+{
+  int col = 0;  // unused
+  grid_adjust(&grid, &row, &col);
+  assert(put_dirty_row == -1);
+  put_dirty_row = row;
+  put_dirty_grid = grid;
+}
+
+void grid_put_schar(ScreenGrid *grid, int row, int col, char_u *schar, int attr)
+{
+  assert(put_dirty_row == row);
+  size_t off = grid->line_offset[row] + (size_t)col;
+  if (grid->attrs[off] != attr || schar_cmp(grid->chars[off], schar)) {
+    schar_copy(grid->chars[off], schar);
+    grid->attrs[off] = attr;
+
+    put_dirty_first = MIN(put_dirty_first, col);
+    // TODO(bfredl): Y U NO DOUBLEWIDTH?
+    put_dirty_last = MAX(put_dirty_last, col + 1);
+  }
+}
+
+/// like grid_puts(), but output "text[len]".  When "len" is -1 output up to
+/// a NUL.
+void grid_puts_len(ScreenGrid *grid, char_u *text, int textlen, int row, int col, int attr)
+{
+  size_t off;
+  char_u *ptr = text;
+  int len = textlen;
+  int c;
+  size_t max_off;
+  int mbyte_blen = 1;
+  int mbyte_cells = 1;
+  int u8c = 0;
+  int u8cc[MAX_MCO];
+  bool clear_next_cell = false;
+  int prev_c = 0;  // previous Arabic character
+  int pc, nc, nc1;
+  int pcc[MAX_MCO];
+  int need_redraw;
+  bool do_flush = false;
+
+  grid_adjust(&grid, &row, &col);
+
+  // Safety check. The check for negative row and column is to fix issue
+  // vim/vim#4102. TODO(neovim): find out why row/col could be negative.
+  if (grid->chars == NULL
+      || row >= grid->Rows || row < 0
+      || col >= grid->Columns || col < 0) {
+    return;
+  }
+
+  if (put_dirty_row == -1) {
+    grid_puts_line_start(grid, row);
+    do_flush = true;
+  } else {
+    if (grid != put_dirty_grid || row != put_dirty_row) {
+      abort();
+    }
+  }
+  off = grid->line_offset[row] + (size_t)col;
+
+  // When drawing over the right half of a double-wide char clear out the
+  // left half.  Only needed in a terminal.
+  if (grid != &default_grid && col == 0 && grid_invalid_row(grid, row)) {
+    // redraw the previous cell, make it empty
+    put_dirty_first = -1;
+    put_dirty_last = MAX(put_dirty_last, 1);
+  }
+
+  max_off = grid->line_offset[row] + (size_t)grid->Columns;
+  while (col < grid->Columns
+         && (len < 0 || (int)(ptr - text) < len)
+         && *ptr != NUL) {
+    c = *ptr;
+    // check if this is the first byte of a multibyte
+    if (len > 0) {
+      mbyte_blen = utfc_ptr2len_len(ptr, (int)((text + len) - ptr));
+    } else {
+      mbyte_blen = utfc_ptr2len((char *)ptr);
+    }
+    if (len >= 0) {
+      u8c = utfc_ptr2char_len(ptr, u8cc, (int)((text + len) - ptr));
+    } else {
+      u8c = utfc_ptr2char(ptr, u8cc);
+    }
+    mbyte_cells = utf_char2cells(u8c);
+    if (p_arshape && !p_tbidi && arabic_char(u8c)) {
+      // Do Arabic shaping.
+      if (len >= 0 && (int)(ptr - text) + mbyte_blen >= len) {
+        // Past end of string to be displayed.
+        nc = NUL;
+        nc1 = NUL;
+      } else {
+        nc = utfc_ptr2char_len(ptr + mbyte_blen, pcc,
+                               (int)((text + len) - ptr - mbyte_blen));
+        nc1 = pcc[0];
+      }
+      pc = prev_c;
+      prev_c = u8c;
+      u8c = arabic_shape(u8c, &c, &u8cc[0], nc, nc1, pc);
+    } else {
+      prev_c = u8c;
+    }
+    if (col + mbyte_cells > grid->Columns) {
+      // Only 1 cell left, but character requires 2 cells:
+      // display a '>' in the last column to avoid wrapping. */
+      c = '>';
+      u8c = '>';
+      u8cc[0] = 0;
+      mbyte_cells = 1;
+    }
+
+    schar_T buf;
+    schar_from_cc(buf, u8c, u8cc);
+
+
+    need_redraw = schar_cmp(grid->chars[off], buf)
+                  || (mbyte_cells == 2 && grid->chars[off + 1][0] != 0)
+                  || grid->attrs[off] != attr
+                  || exmode_active;
+
+    if (need_redraw) {
+      // When at the end of the text and overwriting a two-cell
+      // character with a one-cell character, need to clear the next
+      // cell.  Also when overwriting the left half of a two-cell char
+      // with the right half of a two-cell char.  Do this only once
+      // (utf8_off2cells() may return 2 on the right half).
+      if (clear_next_cell) {
+        clear_next_cell = false;
+      } else if ((len < 0 ? ptr[mbyte_blen] == NUL : ptr + mbyte_blen >= text + len)
+                 && ((mbyte_cells == 1
+                      && grid_off2cells(grid, off, max_off) > 1)
+                     || (mbyte_cells == 2
+                         && grid_off2cells(grid, off, max_off) == 1
+                         && grid_off2cells(grid, off + 1, max_off) > 1))) {
+        clear_next_cell = true;
+      }
+
+      // When at the start of the text and overwriting the right half of a
+      // two-cell character in the same grid, truncate that into a '>'.
+      if (ptr == text && col > 0 && grid->chars[off][0] == 0) {
+        grid->chars[off - 1][0] = '>';
+        grid->chars[off - 1][1] = 0;
+      }
+
+      schar_copy(grid->chars[off], buf);
+      grid->attrs[off] = attr;
+      if (mbyte_cells == 2) {
+        grid->chars[off + 1][0] = 0;
+        grid->attrs[off + 1] = attr;
+      }
+      put_dirty_first = MIN(put_dirty_first, col);
+      put_dirty_last = MAX(put_dirty_last, col + mbyte_cells);
+    }
+
+    off += (size_t)mbyte_cells;
+    col += mbyte_cells;
+    ptr += mbyte_blen;
+    if (clear_next_cell) {
+      // This only happens at the end, display one space next.
+      ptr = (char_u *)" ";
+      len = -1;
+    }
+  }
+
+  if (do_flush) {
+    grid_puts_line_flush(true);
+  }
+}
+
+/// End a group of grid_puts_len calls and send the screen buffer to the UI
+/// layer.
+///
+/// @param set_cursor Move the visible cursor to the end of the changed region.
+///                   This is a workaround for not yet refactored code paths
+///                   and shouldn't be used in new code.
+void grid_puts_line_flush(bool set_cursor)
+{
+  assert(put_dirty_row != -1);
+  if (put_dirty_first < put_dirty_last) {
+    if (set_cursor) {
+      ui_grid_cursor_goto(put_dirty_grid->handle, put_dirty_row,
+                          MIN(put_dirty_last, put_dirty_grid->Columns - 1));
+    }
+    if (!put_dirty_grid->throttled) {
+      ui_line(put_dirty_grid, put_dirty_row, put_dirty_first, put_dirty_last,
+              put_dirty_last, 0, false);
+    } else if (put_dirty_grid->dirty_col) {
+      if (put_dirty_last > put_dirty_grid->dirty_col[put_dirty_row]) {
+        put_dirty_grid->dirty_col[put_dirty_row] = put_dirty_last;
+      }
+    }
+    put_dirty_first = INT_MAX;
+    put_dirty_last = 0;
+  }
+  put_dirty_row = -1;
+  put_dirty_grid = NULL;
+}
+
+/// Fill the grid from "start_row" to "end_row" (exclusive), from "start_col"
+/// to "end_col" (exclusive) with character "c1" in first column followed by
+/// "c2" in the other columns.  Use attributes "attr".
+void grid_fill(ScreenGrid *grid, int start_row, int end_row, int start_col, int end_col, int c1,
+               int c2, int attr)
+{
+  schar_T sc;
+
+  int row_off = 0, col_off = 0;
+  grid_adjust(&grid, &row_off, &col_off);
+  start_row += row_off;
+  end_row += row_off;
+  start_col += col_off;
+  end_col += col_off;
+
+  // safety check
+  if (end_row > grid->Rows) {
+    end_row = grid->Rows;
+  }
+  if (end_col > grid->Columns) {
+    end_col = grid->Columns;
+  }
+
+  // nothing to do
+  if (start_row >= end_row || start_col >= end_col) {
+    return;
+  }
+
+  for (int row = start_row; row < end_row; row++) {
+    // When drawing over the right half of a double-wide char clear
+    // out the left half.  When drawing over the left half of a
+    // double wide-char clear out the right half.  Only needed in a
+    // terminal.
+    if (start_col > 0 && grid_fix_col(grid, start_col, row) != start_col) {
+      grid_puts_len(grid, (char_u *)" ", 1, row, start_col - 1, 0);
+    }
+    if (end_col < grid->Columns
+        && grid_fix_col(grid, end_col, row) != end_col) {
+      grid_puts_len(grid, (char_u *)" ", 1, row, end_col, 0);
+    }
+
+    // if grid was resized (in ext_multigrid mode), the UI has no redraw updates
+    // for the newly resized grid. It is better mark everything as dirty and
+    // send all the updates.
+    int dirty_first = INT_MAX;
+    int dirty_last = 0;
+
+    int col = start_col;
+    schar_from_char(sc, c1);
+    size_t lineoff = grid->line_offset[row];
+    for (col = start_col; col < end_col; col++) {
+      size_t off = lineoff + (size_t)col;
+      if (schar_cmp(grid->chars[off], sc)
+          || grid->attrs[off] != attr) {
+        schar_copy(grid->chars[off], sc);
+        grid->attrs[off] = attr;
+        if (dirty_first == INT_MAX) {
+          dirty_first = col;
+        }
+        dirty_last = col + 1;
+      }
+      if (col == start_col) {
+        schar_from_char(sc, c2);
+      }
+    }
+    if (dirty_last > dirty_first) {
+      // TODO(bfredl): support a cleared suffix even with a batched line?
+      if (put_dirty_row == row) {
+        put_dirty_first = MIN(put_dirty_first, dirty_first);
+        put_dirty_last = MAX(put_dirty_last, dirty_last);
+      } else if (grid->throttled) {
+        // Note: assumes msg_grid is the only throttled grid
+        assert(grid == &msg_grid);
+        int dirty = 0;
+        if (attr != HL_ATTR(HLF_MSG) || c2 != ' ') {
+          dirty = dirty_last;
+        } else if (c1 != ' ') {
+          dirty = dirty_first + 1;
+        }
+        if (grid->dirty_col && dirty > grid->dirty_col[row]) {
+          grid->dirty_col[row] = dirty;
+        }
+      } else {
+        int last = c2 != ' ' ? dirty_last : dirty_first + (c1 != ' ');
+        ui_line(grid, row, dirty_first, last, dirty_last, attr, false);
+      }
+    }
+
+    if (end_col == grid->Columns) {
+      grid->line_wraps[row] = false;
+    }
+  }
+}
+
+/// Check whether the given character needs redrawing:
+/// - the (first byte of the) character is different
+/// - the attributes are different
+/// - the character is multi-byte and the next byte is different
+/// - the character is two cells wide and the second cell differs.
+static int grid_char_needs_redraw(ScreenGrid *grid, size_t off_from, size_t off_to, int cols)
+{
+  return (cols > 0
+          && ((schar_cmp(linebuf_char[off_from], grid->chars[off_to])
+               || linebuf_attr[off_from] != grid->attrs[off_to]
+               || (line_off2cells(linebuf_char, off_from, off_from + (size_t)cols) > 1
+                   && schar_cmp(linebuf_char[off_from + 1],
+                                grid->chars[off_to + 1])))
+              || rdb_flags & RDB_NODELTA));
+}
+
+/// Move one buffered line to the window grid, but only the characters that
+/// have actually changed.  Handle insert/delete character.
+/// "coloff" gives the first column on the grid for this line.
+/// "endcol" gives the columns where valid characters are.
+/// "clear_width" is the width of the window.  It's > 0 if the rest of the line
+/// needs to be cleared, negative otherwise.
+/// "rlflag" is TRUE in a rightleft window:
+///    When TRUE and "clear_width" > 0, clear columns 0 to "endcol"
+///    When FALSE and "clear_width" > 0, clear columns "endcol" to "clear_width"
+/// If "wrap" is true, then hint to the UI that "row" contains a line
+/// which has wrapped into the next row.
+void grid_put_linebuf(ScreenGrid *grid, int row, int coloff, int endcol, int clear_width,
+                      int rlflag, win_T *wp, int bg_attr, bool wrap)
+{
+  size_t max_off_from;
+  size_t max_off_to;
+  int col = 0;
+  bool redraw_this;                         // Does character need redraw?
+  bool redraw_next;                         // redraw_this for next character
+  bool clear_next = false;
+  int char_cells;                           // 1: normal char
+                                            // 2: occupies two display cells
+  int start_dirty = -1, end_dirty = 0;
+
+  // TODO(bfredl): check all callsites and eliminate
+  // Check for illegal row and col, just in case
+  if (row >= grid->Rows) {
+    row = grid->Rows - 1;
+  }
+  if (endcol > grid->Columns) {
+    endcol = grid->Columns;
+  }
+
+  grid_adjust(&grid, &row, &coloff);
+
+  // Safety check. Avoids clang warnings down the call stack.
+  if (grid->chars == NULL || row >= grid->Rows || coloff >= grid->Columns) {
+    DLOG("invalid state, skipped");
+    return;
+  }
+
+  size_t off_from = 0;
+  size_t off_to = grid->line_offset[row] + (size_t)coloff;
+  max_off_from = linebuf_size;
+  max_off_to = grid->line_offset[row] + (size_t)grid->Columns;
+
+  if (rlflag) {
+    // Clear rest first, because it's left of the text.
+    if (clear_width > 0) {
+      while (col <= endcol && grid->chars[off_to][0] == ' '
+             && grid->chars[off_to][1] == NUL
+             && grid->attrs[off_to] == bg_attr) {
+        off_to++;
+        col++;
+      }
+      if (col <= endcol) {
+        grid_fill(grid, row, row + 1, col + coloff, endcol + coloff + 1, ' ', ' ', bg_attr);
+      }
+    }
+    col = endcol + 1;
+    off_to = grid->line_offset[row] + (size_t)col + (size_t)coloff;
+    off_from += (size_t)col;
+    endcol = (clear_width > 0 ? clear_width : -clear_width);
+  }
+
+  if (bg_attr) {
+    for (int c = col; c < endcol; c++) {
+      linebuf_attr[off_from + (size_t)c] =
+        hl_combine_attr(bg_attr, linebuf_attr[off_from + (size_t)c]);
+    }
+  }
+
+  redraw_next = grid_char_needs_redraw(grid, off_from, off_to, endcol - col);
+
+  while (col < endcol) {
+    char_cells = 1;
+    if (col + 1 < endcol) {
+      char_cells = line_off2cells(linebuf_char, off_from, max_off_from);
+    }
+    redraw_this = redraw_next;
+    redraw_next = grid_char_needs_redraw(grid, off_from + (size_t)char_cells,
+                                         off_to + (size_t)char_cells,
+                                         endcol - col - char_cells);
+
+    if (redraw_this) {
+      if (start_dirty == -1) {
+        start_dirty = col;
+      }
+      end_dirty = col + char_cells;
+      // When writing a single-width character over a double-width
+      // character and at the end of the redrawn text, need to clear out
+      // the right half of the old character.
+      // Also required when writing the right half of a double-width
+      // char over the left half of an existing one
+      if (col + char_cells == endcol
+          && ((char_cells == 1
+               && grid_off2cells(grid, off_to, max_off_to) > 1)
+              || (char_cells == 2
+                  && grid_off2cells(grid, off_to, max_off_to) == 1
+                  && grid_off2cells(grid, off_to + 1, max_off_to) > 1))) {
+        clear_next = true;
+      }
+
+      schar_copy(grid->chars[off_to], linebuf_char[off_from]);
+      if (char_cells == 2) {
+        schar_copy(grid->chars[off_to + 1], linebuf_char[off_from + 1]);
+      }
+
+      grid->attrs[off_to] = linebuf_attr[off_from];
+      // For simplicity set the attributes of second half of a
+      // double-wide character equal to the first half.
+      if (char_cells == 2) {
+        grid->attrs[off_to + 1] = linebuf_attr[off_from];
+      }
+    }
+
+    off_to += (size_t)char_cells;
+    off_from += (size_t)char_cells;
+    col += char_cells;
+  }
+
+  if (clear_next) {
+    // Clear the second half of a double-wide character of which the left
+    // half was overwritten with a single-wide character.
+    schar_from_ascii(grid->chars[off_to], ' ');
+    end_dirty++;
+  }
+
+  int clear_end = -1;
+  if (clear_width > 0 && !rlflag) {
+    // blank out the rest of the line
+    // TODO(bfredl): we could cache winline widths
+    while (col < clear_width) {
+      if (grid->chars[off_to][0] != ' '
+          || grid->chars[off_to][1] != NUL
+          || grid->attrs[off_to] != bg_attr) {
+        grid->chars[off_to][0] = ' ';
+        grid->chars[off_to][1] = NUL;
+        grid->attrs[off_to] = bg_attr;
+        if (start_dirty == -1) {
+          start_dirty = col;
+          end_dirty = col;
+        } else if (clear_end == -1) {
+          end_dirty = endcol;
+        }
+        clear_end = col + 1;
+      }
+      col++;
+      off_to++;
+    }
+  }
+
+  if (clear_width > 0 || wp->w_width != grid->Columns) {
+    // If we cleared after the end of the line, it did not wrap.
+    // For vsplit, line wrapping is not possible.
+    grid->line_wraps[row] = false;
+  }
+
+  if (clear_end < end_dirty) {
+    clear_end = end_dirty;
+  }
+  if (start_dirty == -1) {
+    start_dirty = end_dirty;
+  }
+  if (clear_end > start_dirty) {
+    ui_line(grid, row, coloff + start_dirty, coloff + end_dirty, coloff + clear_end,
+            bg_attr, wrap);
+  }
+}
+
+void grid_alloc(ScreenGrid *grid, int rows, int columns, bool copy, bool valid)
+{
+  int new_row;
+  ScreenGrid new = *grid;
+  assert(rows >= 0 && columns >= 0);
+  size_t ncells = (size_t)rows * (size_t)columns;
+  new.chars = xmalloc(ncells * sizeof(schar_T));
+  new.attrs = xmalloc(ncells * sizeof(sattr_T));
+  new.line_offset = xmalloc((size_t)rows * sizeof(*new.line_offset));
+  new.line_wraps = xmalloc((size_t)rows * sizeof(*new.line_wraps));
+
+  new.Rows = rows;
+  new.Columns = columns;
+
+  for (new_row = 0; new_row < new.Rows; new_row++) {
+    new.line_offset[new_row] = (size_t)new_row * (size_t)new.Columns;
+    new.line_wraps[new_row] = false;
+
+    grid_clear_line(&new, new.line_offset[new_row], columns, valid);
+
+    if (copy) {
+      // If the screen is not going to be cleared, copy as much as
+      // possible from the old screen to the new one and clear the rest
+      // (used when resizing the window at the "--more--" prompt or when
+      // executing an external command, for the GUI).
+      if (new_row < grid->Rows && grid->chars != NULL) {
+        int len = MIN(grid->Columns, new.Columns);
+        memmove(new.chars + new.line_offset[new_row],
+                grid->chars + grid->line_offset[new_row],
+                (size_t)len * sizeof(schar_T));
+        memmove(new.attrs + new.line_offset[new_row],
+                grid->attrs + grid->line_offset[new_row],
+                (size_t)len * sizeof(sattr_T));
+      }
+    }
+  }
+  grid_free(grid);
+  *grid = new;
+
+  // Share a single scratch buffer for all grids, by
+  // ensuring it is as wide as the widest grid.
+  if (linebuf_size < (size_t)columns) {
+    xfree(linebuf_char);
+    xfree(linebuf_attr);
+    linebuf_char = xmalloc((size_t)columns * sizeof(schar_T));
+    linebuf_attr = xmalloc((size_t)columns * sizeof(sattr_T));
+    linebuf_size = (size_t)columns;
+  }
+}
+
+void grid_free(ScreenGrid *grid)
+{
+  xfree(grid->chars);
+  xfree(grid->attrs);
+  xfree(grid->line_offset);
+  xfree(grid->line_wraps);
+
+  grid->chars = NULL;
+  grid->attrs = NULL;
+  grid->line_offset = NULL;
+  grid->line_wraps = NULL;
+}
+
+/// Doesn't allow reinit, so must only be called by free_all_mem!
+void grid_free_all_mem(void)
+{
+  grid_free(&default_grid);
+  xfree(linebuf_char);
+  xfree(linebuf_attr);
+}
+
diff --git a/src/nvim/grid.h b/src/nvim/grid.h
new file mode 100644
index 0000000000..12a5bf95bb
--- /dev/null
+++ b/src/nvim/grid.h
@@ -0,0 +1,57 @@
+#ifndef NVIM_GRID_H
+#define NVIM_GRID_H
+
+#include <stdbool.h>
+
+#include "nvim/ascii.h"
+#include "nvim/grid_defs.h"
+#include "nvim/buffer_defs.h"
+
+/// By default, all windows are drawn on a single rectangular grid, represented by
+/// this ScreenGrid instance. In multigrid mode each window will have its own
+/// grid, then this is only used for global screen elements that hasn't been
+/// externalized.
+///
+/// Note: before the screen is initialized and when out of memory these can be
+/// NULL.
+EXTERN ScreenGrid default_grid INIT(= SCREEN_GRID_INIT);
+
+#define DEFAULT_GRID_HANDLE 1  // handle for the default_grid
+
+EXTERN schar_T *linebuf_char INIT(= NULL);
+EXTERN sattr_T *linebuf_attr INIT(= NULL);
+
+// Low-level functions to manipulate individual character cells on the
+// screen grid.
+
+/// Put a ASCII character in a screen cell.
+static inline void schar_from_ascii(char_u *p, const char c)
+{
+  p[0] = (char_u)c;
+  p[1] = 0;
+}
+
+/// Put a unicode character in a screen cell.
+static inline int schar_from_char(char_u *p, int c)
+{
+  int len = utf_char2bytes(c, p);
+  p[len] = NUL;
+  return len;
+}
+
+/// compare the contents of two screen cells.
+static inline int schar_cmp(char_u *sc1, char_u *sc2)
+{
+  return strncmp((char *)sc1, (char *)sc2, sizeof(schar_T));
+}
+
+/// copy the contents of screen cell `sc2` into cell `sc1`
+static inline void schar_copy(char_u *sc1, char_u *sc2)
+{
+  xstrlcpy((char *)sc1, (char *)sc2, sizeof(schar_T));
+}
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "grid.h.generated.h"
+#endif
+#endif  // NVIM_GRID_H
diff --git a/src/nvim/grid_defs.h b/src/nvim/grid_defs.h
index 69eca5d123..2516ea52a7 100644
--- a/src/nvim/grid_defs.h
+++ b/src/nvim/grid_defs.h
@@ -50,7 +50,7 @@ struct ScreenGrid {
 
   schar_T *chars;
   sattr_T *attrs;
-  unsigned *line_offset;
+  size_t *line_offset;
   char_u *line_wraps;
 
   // last column that was drawn (not cleared with the default background).
diff --git a/src/nvim/main.c b/src/nvim/main.c
index 3f9e875d74..9beaa0686f 100644
--- a/src/nvim/main.c
+++ b/src/nvim/main.c
@@ -37,6 +37,7 @@
 # include <locale.h>
 #endif
 #include "nvim/garray.h"
+#include "nvim/grid.h"
 #include "nvim/log.h"
 #include "nvim/mark.h"
 #include "nvim/mbyte.h"
diff --git a/src/nvim/memory.c b/src/nvim/memory.c
index 19144a5dce..8a65188f22 100644
--- a/src/nvim/memory.c
+++ b/src/nvim/memory.c
@@ -696,7 +696,7 @@ void free_all_mem(void)
   }
 
   // free screenlines (can't display anything now!)
-  screen_free_all_mem();
+  grid_free_all_mem();
 
   clear_hl_tables(false);
   list_free_log();
diff --git a/src/nvim/screen.c b/src/nvim/screen.c
index f3e627cc65..968397fbfa 100644
--- a/src/nvim/screen.c
+++ b/src/nvim/screen.c
@@ -130,13 +130,6 @@
 #define MB_FILLER_CHAR '<'  /* character used when a double-width character
                              * doesn't fit. */
 
-// temporary buffer for rendering a single screenline, so it can be
-// compared with previous contents to calculate smallest delta.
-// Per-cell attributes
-static size_t linebuf_size = 0;
-static schar_T *linebuf_char = NULL;
-static sattr_T *linebuf_attr = NULL;
-
 static match_T search_hl;       // used for 'hlsearch' highlight matching
 
 StlClickDefinition *tab_page_click_defs = NULL;
@@ -4221,7 +4214,7 @@ static int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, bool noc
       if (wrap) {
         ScreenGrid *current_grid = grid;
         int current_row = row, dummy_col = 0;  // dummy_col unused
-        screen_adjust_grid(&current_grid, &current_row, &dummy_col);
+        grid_adjust(&current_grid, &current_row, &dummy_col);
 
         // Force a redraw of the first column of the next line.
         current_grid->attrs[current_grid->line_offset[current_row + 1]] = -1;
@@ -4387,25 +4380,6 @@ static int draw_virt_text_item(buf_T *buf, int col, VirtText vt, HlMode hl_mode,
   return col;
 }
 
-/// Determine if dedicated window grid should be used or the default_grid
-///
-/// If UI did not request multigrid support, draw all windows on the
-/// default_grid.
-///
-/// NB: this function can only been used with window grids in a context where
-/// win_grid_alloc already has been called!
-///
-/// If the default_grid is used, adjust window relative positions to global
-/// screen positions.
-void screen_adjust_grid(ScreenGrid **grid, int *row_off, int *col_off)
-{
-  if ((*grid)->target) {
-    *row_off += (*grid)->row_offset;
-    *col_off += (*grid)->col_offset;
-    *grid = (*grid)->target;
-  }
-}
-
 // Return true if CursorLineSign highlight is to be used.
 static bool use_cursor_line_sign(win_T *wp, linenr_T lnum)
 {
@@ -4535,196 +4509,6 @@ static void get_sign_display_info(bool nrcol, win_T *wp, linenr_T lnum, sign_att
 }
 
 
-/*
- * Check whether the given character needs redrawing:
- * - the (first byte of the) character is different
- * - the attributes are different
- * - the character is multi-byte and the next byte is different
- * - the character is two cells wide and the second cell differs.
- */
-static int grid_char_needs_redraw(ScreenGrid *grid, int off_from, int off_to, int cols)
-{
-  return (cols > 0
-          && ((schar_cmp(linebuf_char[off_from], grid->chars[off_to])
-               || linebuf_attr[off_from] != grid->attrs[off_to]
-               || (line_off2cells(linebuf_char, off_from, off_from + cols) > 1
-                   && schar_cmp(linebuf_char[off_from + 1],
-                                grid->chars[off_to + 1])))
-              || rdb_flags & RDB_NODELTA));
-}
-
-/// Move one buffered line to the window grid, but only the characters that
-/// have actually changed.  Handle insert/delete character.
-/// "coloff" gives the first column on the grid for this line.
-/// "endcol" gives the columns where valid characters are.
-/// "clear_width" is the width of the window.  It's > 0 if the rest of the line
-/// needs to be cleared, negative otherwise.
-/// "rlflag" is TRUE in a rightleft window:
-///    When TRUE and "clear_width" > 0, clear columns 0 to "endcol"
-///    When FALSE and "clear_width" > 0, clear columns "endcol" to "clear_width"
-/// If "wrap" is true, then hint to the UI that "row" contains a line
-/// which has wrapped into the next row.
-static void grid_put_linebuf(ScreenGrid *grid, int row, int coloff, int endcol, int clear_width,
-                             int rlflag, win_T *wp, int bg_attr, bool wrap)
-{
-  unsigned off_from;
-  unsigned off_to;
-  unsigned max_off_from;
-  unsigned max_off_to;
-  int col = 0;
-  bool redraw_this;                         // Does character need redraw?
-  bool redraw_next;                         // redraw_this for next character
-  bool clear_next = false;
-  int char_cells;                           // 1: normal char
-                                            // 2: occupies two display cells
-  int start_dirty = -1, end_dirty = 0;
-
-  // TODO(bfredl): check all callsites and eliminate
-  // Check for illegal row and col, just in case
-  if (row >= grid->Rows) {
-    row = grid->Rows - 1;
-  }
-  if (endcol > grid->Columns) {
-    endcol = grid->Columns;
-  }
-
-  screen_adjust_grid(&grid, &row, &coloff);
-
-  // Safety check. Avoids clang warnings down the call stack.
-  if (grid->chars == NULL || row >= grid->Rows || coloff >= grid->Columns) {
-    DLOG("invalid state, skipped");
-    return;
-  }
-
-  off_from = 0;
-  off_to = grid->line_offset[row] + coloff;
-  max_off_from = linebuf_size;
-  max_off_to = grid->line_offset[row] + grid->Columns;
-
-  if (rlflag) {
-    // Clear rest first, because it's left of the text.
-    if (clear_width > 0) {
-      while (col <= endcol && grid->chars[off_to][0] == ' '
-             && grid->chars[off_to][1] == NUL
-             && grid->attrs[off_to] == bg_attr) {
-        ++off_to;
-        ++col;
-      }
-      if (col <= endcol) {
-        grid_fill(grid, row, row + 1, col + coloff, endcol + coloff + 1,
-                  ' ', ' ', bg_attr);
-      }
-    }
-    col = endcol + 1;
-    off_to = grid->line_offset[row] + col + coloff;
-    off_from += col;
-    endcol = (clear_width > 0 ? clear_width : -clear_width);
-  }
-
-  if (bg_attr) {
-    for (int c = col; c < endcol; c++) {
-      linebuf_attr[off_from + c] =
-        hl_combine_attr(bg_attr, linebuf_attr[off_from + c]);
-    }
-  }
-
-  redraw_next = grid_char_needs_redraw(grid, off_from, off_to, endcol - col);
-
-  while (col < endcol) {
-    char_cells = 1;
-    if (col + 1 < endcol) {
-      char_cells = line_off2cells(linebuf_char, off_from, max_off_from);
-    }
-    redraw_this = redraw_next;
-    redraw_next = grid_char_needs_redraw(grid, off_from + char_cells,
-                                         off_to + char_cells,
-                                         endcol - col - char_cells);
-
-    if (redraw_this) {
-      if (start_dirty == -1) {
-        start_dirty = col;
-      }
-      end_dirty = col + char_cells;
-      // When writing a single-width character over a double-width
-      // character and at the end of the redrawn text, need to clear out
-      // the right half of the old character.
-      // Also required when writing the right half of a double-width
-      // char over the left half of an existing one
-      if (col + char_cells == endcol
-          && ((char_cells == 1
-               && grid_off2cells(grid, off_to, max_off_to) > 1)
-              || (char_cells == 2
-                  && grid_off2cells(grid, off_to, max_off_to) == 1
-                  && grid_off2cells(grid, off_to + 1, max_off_to) > 1))) {
-        clear_next = true;
-      }
-
-      schar_copy(grid->chars[off_to], linebuf_char[off_from]);
-      if (char_cells == 2) {
-        schar_copy(grid->chars[off_to + 1], linebuf_char[off_from + 1]);
-      }
-
-      grid->attrs[off_to] = linebuf_attr[off_from];
-      // For simplicity set the attributes of second half of a
-      // double-wide character equal to the first half.
-      if (char_cells == 2) {
-        grid->attrs[off_to + 1] = linebuf_attr[off_from];
-      }
-    }
-
-    off_to += char_cells;
-    off_from += char_cells;
-    col += char_cells;
-  }
-
-  if (clear_next) {
-    // Clear the second half of a double-wide character of which the left
-    // half was overwritten with a single-wide character.
-    schar_from_ascii(grid->chars[off_to], ' ');
-    end_dirty++;
-  }
-
-  int clear_end = -1;
-  if (clear_width > 0 && !rlflag) {
-    // blank out the rest of the line
-    // TODO(bfredl): we could cache winline widths
-    while (col < clear_width) {
-      if (grid->chars[off_to][0] != ' '
-          || grid->chars[off_to][1] != NUL
-          || grid->attrs[off_to] != bg_attr) {
-        grid->chars[off_to][0] = ' ';
-        grid->chars[off_to][1] = NUL;
-        grid->attrs[off_to] = bg_attr;
-        if (start_dirty == -1) {
-          start_dirty = col;
-          end_dirty = col;
-        } else if (clear_end == -1) {
-          end_dirty = endcol;
-        }
-        clear_end = col + 1;
-      }
-      col++;
-      off_to++;
-    }
-  }
-
-  if (clear_width > 0 || wp->w_width != grid->Columns) {
-    // If we cleared after the end of the line, it did not wrap.
-    // For vsplit, line wrapping is not possible.
-    grid->line_wraps[row] = false;
-  }
-
-  if (clear_end < end_dirty) {
-    clear_end = end_dirty;
-  }
-  if (start_dirty == -1) {
-    start_dirty = end_dirty;
-  }
-  if (clear_end > start_dirty) {
-    ui_line(grid, row, coloff + start_dirty, coloff + end_dirty, coloff + clear_end,
-            bg_attr, wrap);
-  }
-}
 
 /*
  * Mirror text "str" for right-left displaying.
@@ -5676,334 +5460,6 @@ static void win_redr_border(win_T *wp)
   }
 }
 
-// Low-level functions to manipulate individual character cells on the
-// screen grid.
-
-/// Put a ASCII character in a screen cell.
-static void schar_from_ascii(char_u *p, const char c)
-{
-  p[0] = c;
-  p[1] = 0;
-}
-
-/// Put a unicode character in a screen cell.
-static int schar_from_char(char_u *p, int c)
-{
-  int len = utf_char2bytes(c, p);
-  p[len] = NUL;
-  return len;
-}
-
-/// Put a unicode char, and up to MAX_MCO composing chars, in a screen cell.
-static int schar_from_cc(char_u *p, int c, int u8cc[MAX_MCO])
-{
-  int len = utf_char2bytes(c, p);
-  for (int i = 0; i < MAX_MCO; i++) {
-    if (u8cc[i] == 0) {
-      break;
-    }
-    len += utf_char2bytes(u8cc[i], p + len);
-  }
-  p[len] = 0;
-  return len;
-}
-
-/// compare the contents of two screen cells.
-static int schar_cmp(char_u *sc1, char_u *sc2)
-{
-  return STRNCMP(sc1, sc2, sizeof(schar_T));
-}
-
-/// copy the contents of screen cell `sc2` into cell `sc1`
-static void schar_copy(char_u *sc1, char_u *sc2)
-{
-  STRLCPY(sc1, sc2, sizeof(schar_T));
-}
-
-static int line_off2cells(schar_T *line, size_t off, size_t max_off)
-{
-  return (off + 1 < max_off && line[off + 1][0] == 0) ? 2 : 1;
-}
-
-/// Return number of display cells for char at grid->chars[off].
-/// We make sure that the offset used is less than "max_off".
-static int grid_off2cells(ScreenGrid *grid, size_t off, size_t max_off)
-{
-  return line_off2cells(grid->chars, off, max_off);
-}
-
-/// Return true if the character at "row"/"col" on the screen is the left side
-/// of a double-width character.
-///
-/// Caller must make sure "row" and "col" are not invalid!
-bool grid_lefthalve(ScreenGrid *grid, int row, int col)
-{
-  screen_adjust_grid(&grid, &row, &col);
-
-  return grid_off2cells(grid, grid->line_offset[row] + col,
-                        grid->line_offset[row] + grid->Columns) > 1;
-}
-
-/// Correct a position on the screen, if it's the right half of a double-wide
-/// char move it to the left half.  Returns the corrected column.
-int grid_fix_col(ScreenGrid *grid, int col, int row)
-{
-  int coloff = 0;
-  screen_adjust_grid(&grid, &row, &coloff);
-
-  col += coloff;
-  if (grid->chars != NULL && col > 0
-      && grid->chars[grid->line_offset[row] + col][0] == 0) {
-    return col - 1 - coloff;
-  }
-  return col - coloff;
-}
-
-/// output a single character directly to the grid
-void grid_putchar(ScreenGrid *grid, int c, int row, int col, int attr)
-{
-  char_u buf[MB_MAXBYTES + 1];
-
-  buf[utf_char2bytes(c, buf)] = NUL;
-  grid_puts(grid, buf, row, col, attr);
-}
-
-/// get a single character directly from grid.chars into "bytes[]".
-/// Also return its attribute in *attrp;
-void grid_getbytes(ScreenGrid *grid, int row, int col, char_u *bytes, int *attrp)
-{
-  unsigned off;
-
-  screen_adjust_grid(&grid, &row, &col);
-
-  // safety check
-  if (grid->chars != NULL && row < grid->Rows && col < grid->Columns) {
-    off = grid->line_offset[row] + col;
-    *attrp = grid->attrs[off];
-    schar_copy(bytes, grid->chars[off]);
-  }
-}
-
-
-/// put string '*text' on the window grid at position 'row' and 'col', with
-/// attributes 'attr', and update chars[] and attrs[].
-/// Note: only outputs within one row, message is truncated at grid boundary!
-/// Note: if grid, row and/or col is invalid, nothing is done.
-void grid_puts(ScreenGrid *grid, char_u *text, int row, int col, int attr)
-{
-  grid_puts_len(grid, text, -1, row, col, attr);
-}
-
-static ScreenGrid *put_dirty_grid = NULL;
-static int put_dirty_row = -1;
-static int put_dirty_first = INT_MAX;
-static int put_dirty_last = 0;
-
-/// Start a group of grid_puts_len calls that builds a single grid line.
-///
-/// Must be matched with a grid_puts_line_flush call before moving to
-/// another line.
-void grid_puts_line_start(ScreenGrid *grid, int row)
-{
-  int col = 0;  // unused
-  screen_adjust_grid(&grid, &row, &col);
-  assert(put_dirty_row == -1);
-  put_dirty_row = row;
-  put_dirty_grid = grid;
-}
-
-void grid_put_schar(ScreenGrid *grid, int row, int col, char_u *schar, int attr)
-{
-  assert(put_dirty_row == row);
-  unsigned int off = grid->line_offset[row] + col;
-  if (grid->attrs[off] != attr || schar_cmp(grid->chars[off], schar)) {
-    schar_copy(grid->chars[off], schar);
-    grid->attrs[off] = attr;
-
-    put_dirty_first = MIN(put_dirty_first, col);
-    // TODO(bfredl): Y U NO DOUBLEWIDTH?
-    put_dirty_last = MAX(put_dirty_last, col + 1);
-  }
-}
-
-/// like grid_puts(), but output "text[len]".  When "len" is -1 output up to
-/// a NUL.
-void grid_puts_len(ScreenGrid *grid, char_u *text, int textlen, int row, int col, int attr)
-{
-  unsigned off;
-  char_u *ptr = text;
-  int len = textlen;
-  int c;
-  unsigned max_off;
-  int mbyte_blen = 1;
-  int mbyte_cells = 1;
-  int u8c = 0;
-  int u8cc[MAX_MCO];
-  int clear_next_cell = FALSE;
-  int prev_c = 0;                       // previous Arabic character
-  int pc, nc, nc1;
-  int pcc[MAX_MCO];
-  int need_redraw;
-  bool do_flush = false;
-
-  screen_adjust_grid(&grid, &row, &col);
-
-  // Safety check. The check for negative row and column is to fix issue
-  // vim/vim#4102. TODO(neovim): find out why row/col could be negative.
-  if (grid->chars == NULL
-      || row >= grid->Rows || row < 0
-      || col >= grid->Columns || col < 0) {
-    return;
-  }
-
-  if (put_dirty_row == -1) {
-    grid_puts_line_start(grid, row);
-    do_flush = true;
-  } else {
-    if (grid != put_dirty_grid || row != put_dirty_row) {
-      abort();
-    }
-  }
-  off = grid->line_offset[row] + col;
-
-  // When drawing over the right half of a double-wide char clear out the
-  // left half.  Only needed in a terminal.
-  if (grid != &default_grid && col == 0 && grid_invalid_row(grid, row)) {
-    // redraw the previous cell, make it empty
-    put_dirty_first = -1;
-    put_dirty_last = MAX(put_dirty_last, 1);
-  }
-
-  max_off = grid->line_offset[row] + grid->Columns;
-  while (col < grid->Columns
-         && (len < 0 || (int)(ptr - text) < len)
-         && *ptr != NUL) {
-    c = *ptr;
-    // check if this is the first byte of a multibyte
-    if (len > 0) {
-      mbyte_blen = utfc_ptr2len_len(ptr, (int)((text + len) - ptr));
-    } else {
-      mbyte_blen = utfc_ptr2len((char *)ptr);
-    }
-    if (len >= 0) {
-      u8c = utfc_ptr2char_len(ptr, u8cc, (int)((text + len) - ptr));
-    } else {
-      u8c = utfc_ptr2char(ptr, u8cc);
-    }
-    mbyte_cells = utf_char2cells(u8c);
-    if (p_arshape && !p_tbidi && arabic_char(u8c)) {
-      // Do Arabic shaping.
-      if (len >= 0 && (int)(ptr - text) + mbyte_blen >= len) {
-        // Past end of string to be displayed.
-        nc = NUL;
-        nc1 = NUL;
-      } else {
-        nc = utfc_ptr2char_len(ptr + mbyte_blen, pcc,
-                               (int)((text + len) - ptr - mbyte_blen));
-        nc1 = pcc[0];
-      }
-      pc = prev_c;
-      prev_c = u8c;
-      u8c = arabic_shape(u8c, &c, &u8cc[0], nc, nc1, pc);
-    } else {
-      prev_c = u8c;
-    }
-    if (col + mbyte_cells > grid->Columns) {
-      // Only 1 cell left, but character requires 2 cells:
-      // display a '>' in the last column to avoid wrapping. */
-      c = '>';
-      u8c = '>';
-      u8cc[0] = 0;
-      mbyte_cells = 1;
-    }
-
-    schar_T buf;
-    schar_from_cc(buf, u8c, u8cc);
-
-
-    need_redraw = schar_cmp(grid->chars[off], buf)
-                  || (mbyte_cells == 2 && grid->chars[off + 1][0] != 0)
-                  || grid->attrs[off] != attr
-                  || exmode_active;
-
-    if (need_redraw) {
-      // When at the end of the text and overwriting a two-cell
-      // character with a one-cell character, need to clear the next
-      // cell.  Also when overwriting the left half of a two-cell char
-      // with the right half of a two-cell char.  Do this only once
-      // (utf8_off2cells() may return 2 on the right half).
-      if (clear_next_cell) {
-        clear_next_cell = false;
-      } else if ((len < 0 ? ptr[mbyte_blen] == NUL
-                          : ptr + mbyte_blen >= text + len)
-                 && ((mbyte_cells == 1
-                      && grid_off2cells(grid, off, max_off) > 1)
-                     || (mbyte_cells == 2
-                         && grid_off2cells(grid, off, max_off) == 1
-                         && grid_off2cells(grid, off + 1, max_off) > 1))) {
-        clear_next_cell = true;
-      }
-
-      // When at the start of the text and overwriting the right half of a
-      // two-cell character in the same grid, truncate that into a '>'.
-      if (ptr == text && col > 0 && grid->chars[off][0] == 0) {
-        grid->chars[off - 1][0] = '>';
-        grid->chars[off - 1][1] = 0;
-      }
-
-      schar_copy(grid->chars[off], buf);
-      grid->attrs[off] = attr;
-      if (mbyte_cells == 2) {
-        grid->chars[off + 1][0] = 0;
-        grid->attrs[off + 1] = attr;
-      }
-      put_dirty_first = MIN(put_dirty_first, col);
-      put_dirty_last = MAX(put_dirty_last, col + mbyte_cells);
-    }
-
-    off += mbyte_cells;
-    col += mbyte_cells;
-    ptr += mbyte_blen;
-    if (clear_next_cell) {
-      // This only happens at the end, display one space next.
-      ptr = (char_u *)" ";
-      len = -1;
-    }
-  }
-
-  if (do_flush) {
-    grid_puts_line_flush(true);
-  }
-}
-
-/// End a group of grid_puts_len calls and send the screen buffer to the UI
-/// layer.
-///
-/// @param set_cursor Move the visible cursor to the end of the changed region.
-///                   This is a workaround for not yet refactored code paths
-///                   and shouldn't be used in new code.
-void grid_puts_line_flush(bool set_cursor)
-{
-  assert(put_dirty_row != -1);
-  if (put_dirty_first < put_dirty_last) {
-    if (set_cursor) {
-      ui_grid_cursor_goto(put_dirty_grid->handle, put_dirty_row,
-                          MIN(put_dirty_last, put_dirty_grid->Columns - 1));
-    }
-    if (!put_dirty_grid->throttled) {
-      ui_line(put_dirty_grid, put_dirty_row, put_dirty_first, put_dirty_last,
-              put_dirty_last, 0, false);
-    } else if (put_dirty_grid->dirty_col) {
-      if (put_dirty_last > put_dirty_grid->dirty_col[put_dirty_row]) {
-        put_dirty_grid->dirty_col[put_dirty_row] = put_dirty_last;
-      }
-    }
-    put_dirty_first = INT_MAX;
-    put_dirty_last = 0;
-  }
-  put_dirty_row = -1;
-  put_dirty_grid = NULL;
-}
 
 /*
  * Prepare for 'hlsearch' highlighting.
@@ -6030,99 +5486,6 @@ static void end_search_hl(void)
 }
 
 
-/// Fill the grid from "start_row" to "end_row" (exclusive), from "start_col"
-/// to "end_col" (exclusive) with character "c1" in first column followed by
-/// "c2" in the other columns.  Use attributes "attr".
-void grid_fill(ScreenGrid *grid, int start_row, int end_row, int start_col, int end_col, int c1,
-               int c2, int attr)
-{
-  schar_T sc;
-
-  int row_off = 0, col_off = 0;
-  screen_adjust_grid(&grid, &row_off, &col_off);
-  start_row += row_off;
-  end_row += row_off;
-  start_col += col_off;
-  end_col += col_off;
-
-  // safety check
-  if (end_row > grid->Rows) {
-    end_row = grid->Rows;
-  }
-  if (end_col > grid->Columns) {
-    end_col = grid->Columns;
-  }
-
-  // nothing to do
-  if (start_row >= end_row || start_col >= end_col) {
-    return;
-  }
-
-  for (int row = start_row; row < end_row; row++) {
-    // When drawing over the right half of a double-wide char clear
-    // out the left half.  When drawing over the left half of a
-    // double wide-char clear out the right half.  Only needed in a
-    // terminal.
-    if (start_col > 0 && grid_fix_col(grid, start_col, row) != start_col) {
-      grid_puts_len(grid, (char_u *)" ", 1, row, start_col - 1, 0);
-    }
-    if (end_col < grid->Columns
-        && grid_fix_col(grid, end_col, row) != end_col) {
-      grid_puts_len(grid, (char_u *)" ", 1, row, end_col, 0);
-    }
-
-    // if grid was resized (in ext_multigrid mode), the UI has no redraw updates
-    // for the newly resized grid. It is better mark everything as dirty and
-    // send all the updates.
-    int dirty_first = INT_MAX;
-    int dirty_last = 0;
-
-    int col = start_col;
-    schar_from_char(sc, c1);
-    int lineoff = grid->line_offset[row];
-    for (col = start_col; col < end_col; col++) {
-      int off = lineoff + col;
-      if (schar_cmp(grid->chars[off], sc)
-          || grid->attrs[off] != attr) {
-        schar_copy(grid->chars[off], sc);
-        grid->attrs[off] = attr;
-        if (dirty_first == INT_MAX) {
-          dirty_first = col;
-        }
-        dirty_last = col + 1;
-      }
-      if (col == start_col) {
-        schar_from_char(sc, c2);
-      }
-    }
-    if (dirty_last > dirty_first) {
-      // TODO(bfredl): support a cleared suffix even with a batched line?
-      if (put_dirty_row == row) {
-        put_dirty_first = MIN(put_dirty_first, dirty_first);
-        put_dirty_last = MAX(put_dirty_last, dirty_last);
-      } else if (grid->throttled) {
-        // Note: assumes msg_grid is the only throttled grid
-        assert(grid == &msg_grid);
-        int dirty = 0;
-        if (attr != HL_ATTR(HLF_MSG) || c2 != ' ') {
-          dirty = dirty_last;
-        } else if (c1 != ' ') {
-          dirty = dirty_first + 1;
-        }
-        if (grid->dirty_col && dirty > grid->dirty_col[row]) {
-          grid->dirty_col[row] = dirty;
-        }
-      } else {
-        int last = c2 != ' ' ? dirty_last : dirty_first + (c1 != ' ');
-        ui_line(grid, row, dirty_first, last, dirty_last, attr, false);
-      }
-    }
-
-    if (end_col == grid->Columns) {
-      grid->line_wraps[row] = false;
-    }
-  }
-}
 
 /// Check if there should be a delay.  Used before clearing or redrawing the
 /// screen or the command line.
@@ -6319,77 +5682,6 @@ retry:
   resizing = false;
 }
 
-void grid_alloc(ScreenGrid *grid, int rows, int columns, bool copy, bool valid)
-{
-  int new_row;
-  ScreenGrid new = *grid;
-  assert(rows >= 0 && columns >= 0);
-  size_t ncells = (size_t)rows * columns;
-  new.chars = xmalloc(ncells * sizeof(schar_T));
-  new.attrs = xmalloc(ncells * sizeof(sattr_T));
-  new.line_offset = xmalloc((size_t)(rows * sizeof(unsigned)));
-  new.line_wraps = xmalloc((size_t)(rows * sizeof(char_u)));
-
-  new.Rows = rows;
-  new.Columns = columns;
-
-  for (new_row = 0; new_row < new.Rows; new_row++) {
-    new.line_offset[new_row] = new_row * new.Columns;
-    new.line_wraps[new_row] = false;
-
-    grid_clear_line(&new, new.line_offset[new_row], columns, valid);
-
-    if (copy) {
-      // If the screen is not going to be cleared, copy as much as
-      // possible from the old screen to the new one and clear the rest
-      // (used when resizing the window at the "--more--" prompt or when
-      // executing an external command, for the GUI).
-      if (new_row < grid->Rows && grid->chars != NULL) {
-        int len = MIN(grid->Columns, new.Columns);
-        memmove(new.chars + new.line_offset[new_row],
-                grid->chars + grid->line_offset[new_row],
-                (size_t)len * sizeof(schar_T));
-        memmove(new.attrs + new.line_offset[new_row],
-                grid->attrs + grid->line_offset[new_row],
-                (size_t)len * sizeof(sattr_T));
-      }
-    }
-  }
-  grid_free(grid);
-  *grid = new;
-
-  // Share a single scratch buffer for all grids, by
-  // ensuring it is as wide as the widest grid.
-  if (linebuf_size < (size_t)columns) {
-    xfree(linebuf_char);
-    xfree(linebuf_attr);
-    linebuf_char = xmalloc(columns * sizeof(schar_T));
-    linebuf_attr = xmalloc(columns * sizeof(sattr_T));
-    linebuf_size = columns;
-  }
-}
-
-void grid_free(ScreenGrid *grid)
-{
-  xfree(grid->chars);
-  xfree(grid->attrs);
-  xfree(grid->line_offset);
-  xfree(grid->line_wraps);
-
-  grid->chars = NULL;
-  grid->attrs = NULL;
-  grid->line_offset = NULL;
-  grid->line_wraps = NULL;
-}
-
-/// Doesn't allow reinit, so must only be called by free_all_mem!
-void screen_free_all_mem(void)
-{
-  grid_free(&default_grid);
-  xfree(linebuf_char);
-  xfree(linebuf_attr);
-}
-
 /// Clear tab_page_click_defs table
 ///
 /// @param[out]  tpcd  Table to clear.
@@ -6457,28 +5749,6 @@ void screenclear(void)
   }
 }
 
-/// clear a line in the grid starting at "off" until "width" characters
-/// are cleared.
-void grid_clear_line(ScreenGrid *grid, unsigned off, int width, bool valid)
-{
-  for (int col = 0; col < width; col++) {
-    schar_from_ascii(grid->chars[off + col], ' ');
-  }
-  int fill = valid ? 0 : -1;
-  (void)memset(grid->attrs + off, fill, (size_t)width * sizeof(sattr_T));
-}
-
-void grid_invalidate(ScreenGrid *grid)
-{
-  (void)memset(grid->attrs, -1, sizeof(sattr_T) * grid->Rows * grid->Columns);
-}
-
-bool grid_invalid_row(ScreenGrid *grid, int row)
-{
-  return grid->attrs[grid->line_offset[row]] < 0;
-}
-
-
 /// Copy part of a grid line for vertically split window.
 static void linecopy(ScreenGrid *grid, int to, int from, int col, int width)
 {
@@ -6510,7 +5780,7 @@ void setcursor(void)
                 && vim_isprintc(gchar_cursor())) ? 2 : 1);
     }
 
-    screen_adjust_grid(&grid, &row, &col);
+    grid_adjust(&grid, &row, &col);
     ui_grid_cursor_goto(grid->handle, row, col);
   }
 }
@@ -6563,7 +5833,7 @@ void grid_ins_lines(ScreenGrid *grid, int row, int line_count, int end, int col,
   unsigned temp;
 
   int row_off = 0;
-  screen_adjust_grid(&grid, &row_off, &col);
+  grid_adjust(&grid, &row_off, &col);
   row += row_off;
   end += row_off;
 
@@ -6612,7 +5882,7 @@ void grid_del_lines(ScreenGrid *grid, int row, int line_count, int end, int col,
   unsigned temp;
 
   int row_off = 0;
-  screen_adjust_grid(&grid, &row_off, &col);
+  grid_adjust(&grid, &row_off, &col);
   row += row_off;
   end += row_off;
 
diff --git a/src/nvim/screen.h b/src/nvim/screen.h
index 33b713e756..3afbaa5eb6 100644
--- a/src/nvim/screen.h
+++ b/src/nvim/screen.h
@@ -4,7 +4,7 @@
 #include <stdbool.h>
 
 #include "nvim/buffer_defs.h"
-#include "nvim/grid_defs.h"
+#include "nvim/grid.h"
 #include "nvim/pos.h"
 #include "nvim/types.h"
 
@@ -27,17 +27,6 @@ typedef enum {
   WC_BOTTOM_RIGHT,
 } WindowCorner;
 
-/// By default, all windows are drawn on a single rectangular grid, represented by
-/// this ScreenGrid instance. In multigrid mode each window will have its own
-/// grid, then this is only used for global screen elements that hasn't been
-/// externalized.
-///
-/// Note: before the screen is initialized and when out of memory these can be
-/// NULL.
-EXTERN ScreenGrid default_grid INIT(= SCREEN_GRID_INIT);
-
-#define DEFAULT_GRID_HANDLE 1  // handle for the default_grid
-
 // Maximum columns for terminal highlight attributes
 #define TERM_ATTRS_MAX 1024
 
diff --git a/src/nvim/ui_compositor.c b/src/nvim/ui_compositor.c
index 60ad8609a4..f76158ff12 100644
--- a/src/nvim/ui_compositor.c
+++ b/src/nvim/ui_compositor.c
@@ -635,7 +635,7 @@ static void ui_comp_grid_scroll(UI *ui, Integer grid, Integer top, Integer bot,
       // ideally win_update() should keep track of this itself and not scroll
       // the invalid space.
       if (curgrid->attrs[curgrid->line_offset[r - curgrid->comp_row]
-                         + left - curgrid->comp_col] >= 0) {
+                         + (size_t)left - (size_t)curgrid->comp_col] >= 0) {
         compose_line(r, left, right, 0);
       }
     }
diff --git a/src/nvim/window.c b/src/nvim/window.c
index 9cf1e3eca1..bbae2bfaaf 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -806,7 +806,7 @@ void win_config_float(win_T *wp, FloatConfig fconfig)
       col += parent->w_wincol;
       ScreenGrid *grid = &parent->w_grid;
       int row_off = 0, col_off = 0;
-      screen_adjust_grid(&grid, &row_off, &col_off);
+      grid_adjust(&grid, &row_off, &col_off);
       row += row_off;
       col += col_off;
     }
@@ -877,7 +877,7 @@ void ui_ext_win_position(win_T *wp)
       if (win) {
         grid = &win->w_grid;
         int row_off = 0, col_off = 0;
-        screen_adjust_grid(&grid, &row_off, &col_off);
+        grid_adjust(&grid, &row_off, &col_off);
         row += row_off;
         col += col_off;
         if (c.bufpos.lnum >= 0) {