perf(screen): reduce cursorline redrawing when jumping around

vim-patch:8.2.4614: redrawing too much when 'cursorline' is set

Problem:    Redrawing too much when 'cursorline' is set and jumping around.
Solution:   Rely on win_update() to redraw the current and previous cursor
            line, do not mark lines as modified. (closes vim/vim#9996)
c20e46a4e3

This doesn't match the patch exactly, because I missed some lines when
porting patch 8.1.2029, and these lines were removed in this patch.

This also makes win_update() always update for 'concealcursor' like how
it always updates for 'cursorline', as 'cursorline' and 'concealcursor'
redrawing logic has been unified in Nvim.

As redrawing for 'cursorline' now always only requires VALID redraw
type, it is no longer necessary to call redraw_for_cursorline() in
nvim_win_set_cursor().

src/nvim/api/window.c

@@ -119,7 +119,6 @@ void nvim_win_set_cursor(Window window, ArrayOf(Integer, 2) pos, Error *err)
   update_topline_win(win);
 
   redraw_later(win, VALID);
-  redraw_for_cursorline(win);
   win->w_redr_status = true;
 }
 

src/nvim/move.c

@@ -95,11 +95,6 @@ static void comp_botline(win_T *wp)
   win_check_anchored_floats(wp);
 }
 
-void reset_cursorline(void)
-{
-  curwin->w_last_cursorline = 0;
-}
-
 // Redraw when w_cline_row changes and 'relativenumber' or 'cursorline' is set.
 void redraw_for_cursorline(win_T *wp)
   FUNC_ATTR_NONNULL_ALL
@@ -107,22 +102,9 @@ void redraw_for_cursorline(win_T *wp)
   if ((wp->w_p_rnu || win_cursorline_standout(wp))
       && (wp->w_valid & VALID_CROW) == 0
       && !pum_visible()) {
-    if (wp->w_p_rnu) {
+    // win_line() will redraw the number column and cursorline only.
-      // win_line() will redraw the number column only.
     redraw_later(wp, VALID);
   }
-    if (win_cursorline_standout(wp)) {
-      if (wp->w_redr_type <= VALID && wp->w_last_cursorline != 0) {
-        // "w_last_cursorline" may be outdated, worst case we redraw
-        // too much.  This is optimized for moving the cursor around in
-        // the current window.
-        redrawWinline(wp, wp->w_last_cursorline);
-        redrawWinline(wp, wp->w_cursor.lnum);
-      } else {
-        redraw_later(wp, SOME_VALID);
-      }
-    }
-  }
 }
 
 /*

src/nvim/option.c

@@ -3962,11 +3962,8 @@ static char *set_bool_option(const int opt_idx, char_u *const varp, const int va
   } else if ((int *)varp == &p_lnr) {
     // 'langnoremap' -> !'langremap'
     p_lrm = !p_lnr;
-  } else if ((int *)varp == &curwin->w_p_cul && !value && old_value) {
-    // 'cursorline'
-    reset_cursorline();
-    // 'undofile'
   } else if ((int *)varp == &curbuf->b_p_udf || (int *)varp == &p_udf) {
+    // 'undofile'
     // Only take action when the option was set. When reset we do not
     // delete the undo file, the option may be set again without making
     // any changes in between.

src/nvim/screen.c

@@ -737,9 +737,6 @@ static void win_update(win_T *wp, DecorProviders *providers)
 #define DID_FOLD 3      // updated a folded line
   int did_update = DID_NONE;
   linenr_T syntax_last_parsed = 0;              // last parsed text line
-  // remember the current w_last_cursorline, it changes when drawing the new
-  // cursor line
-  linenr_T last_cursorline = wp->w_last_cursorline;
   linenr_T mod_top = 0;
   linenr_T mod_bot = 0;
   int save_got_int;
@@ -1326,6 +1323,8 @@ static void win_update(win_T *wp, DecorProviders *providers)
   DecorProviders line_providers;
   decor_providers_invoke_win(wp, providers, &line_providers, &provider_err);
 
+  bool cursorline_standout = win_cursorline_standout(wp);
+
   for (;;) {
     /* stop updating when reached the end of the window (check for _past_
      * the end of the window is at the end of the loop) */
@@ -1370,8 +1369,8 @@ static void win_update(win_T *wp, DecorProviders *providers)
                         // if lines were inserted or deleted
                         || (wp->w_match_head != NULL
                             && buf->b_mod_xlines != 0)))))
-        || (wp->w_p_cul && (lnum == wp->w_cursor.lnum
+        || (cursorline_standout && lnum == wp->w_cursor.lnum)
-                            || lnum == last_cursorline))) {
+        || lnum == wp->w_last_cursorline) {
       if (lnum == mod_top) {
         top_to_mod = false;
       }
@@ -1604,6 +1603,9 @@ static void win_update(win_T *wp, DecorProviders *providers)
    * End of loop over all window lines.
    */
 
+  // Now that the window has been redrawn with the old and new cursor line,
+  // update w_last_cursorline.
+  wp->w_last_cursorline = cursorline_standout ? wp->w_cursor.lnum : 0;
 
   if (idx > wp->w_lines_valid) {
     wp->w_lines_valid = idx;
@@ -2383,8 +2385,6 @@ static int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, bool noc
       }
       area_highlighting = true;
     }
-    // Update w_last_cursorline even if Visual mode is active.
-    wp->w_last_cursorline = wp->w_cursor.lnum;
   }
 
   memset(sattrs, 0, sizeof(sattrs));