multigrid: don't clear window grids on resize

Instead define that the shared top-left part of the grid is preserved.

2 files changed, 12 insertions, 14 deletions

diff --git a/src/nvim/move.c b/src/nvim/move.c
index 4267b4bb61..ebbcd1b853 100644
--- a/src/nvim/move.c
+++ b/src/nvim/move.c
@@ -529,7 +529,7 @@ int cursor_valid(void)
  */
 void validate_cursor(void)
 {
-  win_grid_alloc(curwin, true);  // we need to have w_grid.Rows/Columns updated
+  win_grid_alloc(curwin, false);  // we need to have w_grid.Rows/Columns updated
   check_cursor_moved(curwin);
   if ((curwin->w_valid & (VALID_WCOL|VALID_WROW)) != (VALID_WCOL|VALID_WROW))
     curs_columns(true);
diff --git a/src/nvim/screen.c b/src/nvim/screen.c
index 17006c2954..953aea0094 100644
--- a/src/nvim/screen.c
+++ b/src/nvim/screen.c
@@ -679,7 +679,7 @@ static void win_update(win_T *wp)
 
   type = wp->w_redr_type;
 
-  win_grid_alloc(wp, true);
+  win_grid_alloc(wp, false);
 
   if (type >= NOT_VALID) {
     wp->w_redr_status = true;
@@ -5897,8 +5897,8 @@ void grid_fill(ScreenGrid *grid, int start_row, int end_row, int start_col,
     // 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 = grid->was_resized ? start_col : INT_MAX;
-    int dirty_last = grid->was_resized ? grid->Columns : 0;
+    int dirty_first = INT_MAX;
+    int dirty_last = 0;
 
     int col = start_col;
     schar_from_char(sc, c1);
@@ -5912,9 +5912,7 @@ void grid_fill(ScreenGrid *grid, int start_row, int end_row, int start_col,
         if (dirty_first == INT_MAX) {
           dirty_first = col;
         }
-        if (!grid->was_resized) {
-          dirty_last = col+1;
-        }
+        dirty_last = col+1;
       }
       if (col == start_col) {
         schar_from_char(sc, c2);
@@ -6152,7 +6150,7 @@ retry:
 
 void grid_alloc(ScreenGrid *grid, int rows, int columns, bool copy)
 {
-  int new_row, old_row;
+  int new_row;
   ScreenGrid new = *grid;
 
   size_t ncells = (size_t)((rows+1) * columns);
@@ -6168,21 +6166,20 @@ void grid_alloc(ScreenGrid *grid, int rows, int columns, bool copy)
     new.LineOffset[new_row] = new_row * new.Columns;
     new.LineWraps[new_row] = false;
 
-    grid_clear_line(&new, 0, columns);
+    grid_clear_line(&new, new.LineOffset[new_row], columns, true);
 
     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).
-      old_row = new_row + (grid->Rows - new.Rows);
-      if (old_row >= 0 && grid->ScreenLines != NULL) {
+      if (new_row < grid->Rows && grid->ScreenLines != NULL) {
         int len = MIN(grid->Columns, new.Columns);
         memmove(new.ScreenLines + new.LineOffset[new_row],
-                grid->ScreenLines + grid->LineOffset[old_row],
+                grid->ScreenLines + grid->LineOffset[new_row],
                 (size_t)len * sizeof(schar_T));
         memmove(new.ScreenAttrs + new.LineOffset[new_row],
-                grid->ScreenAttrs + grid->LineOffset[old_row],
+                grid->ScreenAttrs + grid->LineOffset[new_row],
                 (size_t)len * sizeof(sattr_T));
       }
     }
@@ -6281,7 +6278,8 @@ static void screenclear2(void)
 
 /// clear a line in the grid starting at "off" until "width" characters
 /// are cleared.
-static void grid_clear_line(ScreenGrid *grid, unsigned off, int width, bool valid)
+static void grid_clear_line(ScreenGrid *grid, unsigned off, int width,
+                            bool valid)
 {
   for (int col = 0; col < width; col++) {
     schar_from_ascii(grid->ScreenLines[off + col], ' ');