perf(redraw): reduce redraw with undo and extmarks or 'spell' (#27442)

vim-patch:9.1.0100: Redrawing can be improved with undo and 'spell'

Problem:  When undoing with 'spell', redrawWinline() is called after
          changed_lines(), while later win_update() sets redraw type to
          UPD_NOT_VALID, even though w_redraw_top and w_redraw_bot are
          still valid.
Solution: Only set redraw type to UPD_NOT_VALID when inserting/deleting
          lines after parts of window has pending redraw, i.e., when
          changed_lines() is called after redrawWinline().
          (zeertzjq)

closes: vim/vim#14019

f2d90a3511

src/nvim/change.c

@@ -326,6 +326,13 @@ static void changed_common(buf_T *buf, linenr_T lnum, colnr_T col, linenr_T lnum
         wp->w_redr_type = UPD_VALID;
       }
 
+      // When inserting/deleting lines and the window has specific lines
+      // to be redrawn, w_redraw_top and w_redraw_bot may now be invalid,
+      // so just redraw everything.
+      if (xtra != 0 && wp->w_redraw_top != 0) {
+        redraw_later(wp, UPD_NOT_VALID);
+      }
+
       linenr_T last = lnume + xtra - 1;  // last line after the change
 
       // Reset "w_skipcol" if the topline length has become smaller to

src/nvim/drawscreen.c

@@ -1427,7 +1427,7 @@ static void draw_sep_connectors_win(win_T *wp)
 ///                     - if wp->w_buffer->b_mod_set set, update lines between
 ///                       b_mod_top and b_mod_bot.
 ///                     - if wp->w_redraw_top non-zero, redraw lines between
-///                       wp->w_redraw_top and wp->w_redr_bot.
+///                       wp->w_redraw_top and wp->w_redraw_bot.
 ///                     - continue redrawing when syntax status is invalid.
 ///                  4. if scrolled up, update lines at the bottom.
 /// This results in three areas that may need updating:
@@ -1537,13 +1537,6 @@ static void win_update(win_T *wp)
   if (wp->w_nrwidth != nrwidth_new) {
     type = UPD_NOT_VALID;
     wp->w_nrwidth = nrwidth_new;
-  } else if (buf->b_mod_set
-             && buf->b_mod_xlines != 0
-             && wp->w_redraw_top != 0) {
-    // When there are both inserted/deleted lines and specific lines to be
-    // redrawn, w_redraw_top and w_redraw_bot may be invalid, just redraw
-    // everything (only happens when redrawing is off for while).
-    type = UPD_NOT_VALID;
   } else {
     // Set mod_top to the first line that needs displaying because of
     // changes.  Set mod_bot to the first line after the changes.