feat(messages)!: graduate the 'msgsep' feature

The old behaviour (e.g. via `set display-=msgsep`) will not be available.
Assuming that messages always are being drawn on msg_grid
(or not drawn at all, and forwarded to `ext_messages` enabled UI)
will allows some simplifcations and enhancements moving forward.

6 files changed, 60 insertions, 115 deletions

diff --git a/src/nvim/drawscreen.c b/src/nvim/drawscreen.c
index 7ad4193058..15a7294496 100644
--- a/src/nvim/drawscreen.c
+++ b/src/nvim/drawscreen.c
@@ -428,74 +428,41 @@ int update_screen(void)
                         msg_grid.cols, false);
       }
     }
-    if (msg_use_msgsep()) {
-      msg_grid.throttled = false;
-      bool was_invalidated = false;
-
-      // UPD_CLEAR is already handled
-      if (type == UPD_NOT_VALID && !ui_has(kUIMultigrid) && msg_scrolled) {
-        was_invalidated = ui_comp_set_screen_valid(false);
-        for (int i = valid; i < Rows - p_ch; i++) {
-          grid_clear_line(&default_grid, default_grid.line_offset[i],
-                          Columns, false);
+    msg_grid.throttled = false;
+    bool was_invalidated = false;
+
+    // UPD_CLEAR is already handled
+    if (type == UPD_NOT_VALID && !ui_has(kUIMultigrid) && msg_scrolled) {
+      was_invalidated = ui_comp_set_screen_valid(false);
+      for (int i = valid; i < Rows - p_ch; i++) {
+        grid_clear_line(&default_grid, default_grid.line_offset[i],
+                        Columns, false);
+      }
+      FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
+        if (wp->w_floating) {
+          continue;
         }
-        FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
-          if (wp->w_floating) {
-            continue;
-          }
-          if (W_ENDROW(wp) > valid) {
-            // TODO(bfredl): too pessimistic. type could be UPD_NOT_VALID
-            // only because windows that are above the separator.
-            wp->w_redr_type = MAX(wp->w_redr_type, UPD_NOT_VALID);
-          }
-          if (!is_stl_global && W_ENDROW(wp) + wp->w_status_height > valid) {
-            wp->w_redr_status = true;
-          }
+        if (W_ENDROW(wp) > valid) {
+          // TODO(bfredl): too pessimistic. type could be UPD_NOT_VALID
+          // only because windows that are above the separator.
+          wp->w_redr_type = MAX(wp->w_redr_type, UPD_NOT_VALID);
         }
-        if (is_stl_global && Rows - p_ch - 1 > valid) {
-          curwin->w_redr_status = true;
+        if (!is_stl_global && W_ENDROW(wp) + wp->w_status_height > valid) {
+          wp->w_redr_status = true;
         }
       }
-      msg_grid_set_pos(Rows - (int)p_ch, false);
-      msg_grid_invalid = false;
-      if (was_invalidated) {
-        // screen was only invalid for the msgarea part.
-        // @TODO(bfredl): using the same "valid" flag
-        // for both messages and floats moving is bit of a mess.
-        ui_comp_set_screen_valid(true);
-      }
-    } else if (type != UPD_CLEAR) {
-      if (msg_scrolled > Rows - 5) {  // redrawing is faster
-        type = UPD_NOT_VALID;
-      } else {
-        check_for_delay(false);
-        grid_ins_lines(&default_grid, 0, msg_scrolled, Rows, 0, Columns);
-        FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
-          if (wp->w_floating) {
-            continue;
-          }
-          if (wp->w_winrow < msg_scrolled) {
-            if (W_ENDROW(wp) > msg_scrolled
-                && wp->w_redr_type < UPD_REDRAW_TOP
-                && wp->w_lines_valid > 0
-                && wp->w_topline == wp->w_lines[0].wl_lnum) {
-              wp->w_upd_rows = msg_scrolled - wp->w_winrow;
-              wp->w_redr_type = UPD_REDRAW_TOP;
-            } else {
-              wp->w_redr_type = UPD_NOT_VALID;
-              if (wp->w_winrow + wp->w_winbar_height <= msg_scrolled) {
-                wp->w_redr_status = true;
-              }
-            }
-          }
-        }
-        if (is_stl_global && Rows - p_ch - 1 <= msg_scrolled) {
-          curwin->w_redr_status = true;
-        }
-        redraw_cmdline = true;
-        redraw_tabline = true;
+      if (is_stl_global && Rows - p_ch - 1 > valid) {
+        curwin->w_redr_status = true;
       }
     }
+    msg_grid_set_pos(Rows - (int)p_ch, false);
+    msg_grid_invalid = false;
+    if (was_invalidated) {
+      // screen was only invalid for the msgarea part.
+      // @TODO(bfredl): using the same "valid" flag
+      // for both messages and floats moving is bit of a mess.
+      ui_comp_set_screen_valid(true);
+    }
     msg_scrolled = 0;
     msg_scrolled_at_flush = 0;
     need_wait_return = false;
@@ -526,9 +493,8 @@ int update_screen(void)
     default_grid.valid = true;
   }
 
-  // After disabling msgsep the grid might not have been deallocated yet,
-  // hence we also need to check msg_grid.chars
-  if (type == UPD_NOT_VALID && (msg_use_grid() || msg_grid.chars)) {
+  // might need to clear space on default_grid for the message area.
+  if (type == UPD_NOT_VALID && clear_cmdline && !ui_has(kUIMessages)) {
     grid_fill(&default_grid, Rows - (int)p_ch, Rows, 0, Columns, ' ', ' ', 0);
   }
 
diff --git a/src/nvim/ex_docmd.c b/src/nvim/ex_docmd.c
index 73ce92c823..5591b5f55d 100644
--- a/src/nvim/ex_docmd.c
+++ b/src/nvim/ex_docmd.c
@@ -6124,9 +6124,6 @@ static void ex_redrawstatus(exarg_T *eap)
   } else {
     status_redraw_curbuf();
   }
-  if (msg_scrolled && !msg_use_msgsep() && (State & MODE_CMDLINE)) {
-    return;  // redraw later
-  }
 
   RedrawingDisabled = 0;
   p_lz = false;
diff --git a/src/nvim/ex_getln.c b/src/nvim/ex_getln.c
index d1d0fe3f2a..6883dc5b14 100644
--- a/src/nvim/ex_getln.c
+++ b/src/nvim/ex_getln.c
@@ -775,7 +775,7 @@ static uint8_t *command_line_enter(int firstc, long count, int indent, bool init
 
   // Redraw the statusline in case it uses the current mode using the mode()
   // function.
-  if (!cmd_silent && (msg_scrolled == 0 || msg_use_msgsep())) {
+  if (!cmd_silent) {
     bool found_one = false;
 
     FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
diff --git a/src/nvim/message.c b/src/nvim/message.c
index d6835e04eb..fef5055c84 100644
--- a/src/nvim/message.c
+++ b/src/nvim/message.c
@@ -153,8 +153,7 @@ void msg_grid_set_pos(int row, bool scrolled)
 
 bool msg_use_grid(void)
 {
-  return default_grid.chars && msg_use_msgsep()
-         && !ui_has(kUIMessages);
+  return default_grid.chars && !ui_has(kUIMessages);
 }
 
 void msg_grid_validate(void)
@@ -172,19 +171,17 @@ void msg_grid_validate(void)
     xfree(msg_grid.dirty_col);
     msg_grid.dirty_col = xcalloc((size_t)Rows, sizeof(*msg_grid.dirty_col));
 
-    // Tricky: allow resize while pager is active
-    int pos = msg_scrolled ? msg_grid_pos : max_rows;
+    // Tricky: allow resize while pager or ex mode is active
+    int pos = MAX(max_rows - msg_scrolled, 0);
+    msg_grid.throttled = false;  // don't throttle in 'cmdheight' area
+    msg_grid_set_pos(pos, msg_scrolled);
     ui_comp_put_grid(&msg_grid, pos, 0, msg_grid.rows, msg_grid.cols,
                      false, true);
     ui_call_grid_resize(msg_grid.handle, msg_grid.cols, msg_grid.rows);
 
-    msg_grid.throttled = false;  // don't throttle in 'cmdheight' area
     msg_scrolled_at_flush = msg_scrolled;
     msg_grid.focusable = false;
     msg_grid_adj.target = &msg_grid;
-    if (!msg_scrolled) {
-      msg_grid_set_pos(max_rows, false);
-    }
   } else if (!should_alloc && msg_grid.chars) {
     ui_comp_remove_grid(&msg_grid);
     grid_free(&msg_grid);
@@ -2336,13 +2333,6 @@ int msg_scrollsize(void)
   return msg_scrolled + (int)p_ch + ((p_ch > 0 || msg_scrolled > 1) ? 1 : 0);
 }
 
-bool msg_use_msgsep(void)
-{
-  // the full-screen scroll behavior doesn't really make sense with
-  // 'ext_multigrid'
-  return (dy_flags & DY_MSGSEP) || p_ch == 0 || ui_has(kUIMultigrid);
-}
-
 bool msg_do_throttle(void)
 {
   return msg_use_grid() && !(rdb_flags & RDB_NOTHROTTLE);
@@ -2355,23 +2345,19 @@ void msg_scroll_up(bool may_throttle, bool zerocmd)
     msg_grid.throttled = true;
   }
   msg_did_scroll = true;
-  if (msg_use_msgsep()) {
-    if (msg_grid_pos > 0) {
-      msg_grid_set_pos(msg_grid_pos - 1, !zerocmd);
-
-      // When displaying the first line with cmdheight=0, we need to draw over
-      // the existing last line of the screen.
-      if (zerocmd && msg_grid.chars) {
-        grid_clear_line(&msg_grid, msg_grid.line_offset[0], msg_grid.cols, false);
-      }
-    } else {
-      grid_del_lines(&msg_grid, 0, 1, msg_grid.rows, 0, msg_grid.cols);
-      memmove(msg_grid.dirty_col, msg_grid.dirty_col + 1,
-              (size_t)(msg_grid.rows - 1) * sizeof(*msg_grid.dirty_col));
-      msg_grid.dirty_col[msg_grid.rows - 1] = 0;
+  if (msg_grid_pos > 0) {
+    msg_grid_set_pos(msg_grid_pos - 1, !zerocmd);
+
+    // When displaying the first line with cmdheight=0, we need to draw over
+    // the existing last line of the screen.
+    if (zerocmd && msg_grid.chars) {
+      grid_clear_line(&msg_grid, msg_grid.line_offset[0], msg_grid.cols, false);
     }
   } else {
-    grid_del_lines(&msg_grid_adj, 0, 1, Rows, 0, Columns);
+    grid_del_lines(&msg_grid, 0, 1, msg_grid.rows, 0, msg_grid.cols);
+    memmove(msg_grid.dirty_col, msg_grid.dirty_col + 1,
+            (size_t)(msg_grid.rows - 1) * sizeof(*msg_grid.dirty_col));
+    msg_grid.dirty_col[msg_grid.rows - 1] = 0;
   }
 
   grid_fill(&msg_grid_adj, Rows - 1, Rows, 0, Columns, ' ', ' ', HL_ATTR(HLF_MSG));
@@ -2436,21 +2422,17 @@ void msg_reset_scroll(void)
   }
   // TODO(bfredl): some duplicate logic with update_screen(). Later on
   // we should properly disentangle message clear with full screen redraw.
-  if (msg_use_grid()) {
-    msg_grid.throttled = false;
-    // TODO(bfredl): risk for extra flicker i e with
-    // "nvim -o has_swap also_has_swap"
-    msg_grid_set_pos(Rows - (int)p_ch, false);
-    clear_cmdline = true;
-    if (msg_grid.chars) {
-      // non-displayed part of msg_grid is considered invalid.
-      for (int i = 0; i < MIN(msg_scrollsize(), msg_grid.rows); i++) {
-        grid_clear_line(&msg_grid, msg_grid.line_offset[i],
-                        msg_grid.cols, false);
-      }
+  msg_grid.throttled = false;
+  // TODO(bfredl): risk for extra flicker i e with
+  // "nvim -o has_swap also_has_swap"
+  msg_grid_set_pos(Rows - (int)p_ch, false);
+  clear_cmdline = true;
+  if (msg_grid.chars) {
+    // non-displayed part of msg_grid is considered invalid.
+    for (int i = 0; i < MIN(msg_scrollsize(), msg_grid.rows); i++) {
+      grid_clear_line(&msg_grid, msg_grid.line_offset[i],
+                      msg_grid.cols, false);
     }
-  } else {
-    redraw_all_later(UPD_NOT_VALID);
   }
   msg_scrolled = 0;
   msg_scrolled_at_flush = 0;
diff --git a/src/nvim/option_defs.h b/src/nvim/option_defs.h
index e27607d7a8..4e48c992c2 100644
--- a/src/nvim/option_defs.h
+++ b/src/nvim/option_defs.h
@@ -492,7 +492,7 @@ EXTERN unsigned dy_flags;
 #define DY_LASTLINE             0x001
 #define DY_TRUNCATE             0x002
 #define DY_UHEX                 0x004
-// code should use msg_use_msgsep() to check if msgsep is active
+// legacy flag, not used
 #define DY_MSGSEP               0x008
 EXTERN int p_ed;                // 'edcompatible'
 EXTERN char *p_ead;             // 'eadirection'
diff --git a/src/nvim/options.lua b/src/nvim/options.lua
index 048d9e112b..82fd0d7428 100644
--- a/src/nvim/options.lua
+++ b/src/nvim/options.lua
@@ -660,7 +660,7 @@ return {
       deny_duplicates=true,
       redraw={'all_windows'},
       varname='p_dy',
-      defaults={if_true="lastline,msgsep"}
+      defaults={if_true="lastline"}
     },
     {
       full_name='eadirection', abbreviation='ead',