win_line: update w_last_cursorline always

Vim patch 8.1.0856 (54d9ea6) caused a performance regression in Neovim,
when `set conceallevel=1 nocursorline` was used, since then due to
refactoring in 23c71d5 `w_last_cursorline` would never get updated
anymore.

Adds/uses `redrawdebug+=nodelta` for testing this.

Fixes https://github.com/neovim/neovim/issues/11100.
Closes https://github.com/neovim/neovim/pull/11101.

runtime/doc/options.txt

@@ -4588,9 +4588,11 @@ A jump table for the options with a short description can be found at |Q_op|.
 			larger scroll when drawing the message area (with
 			'display' msgsep flag active).
 	    invalid	Enable stricter checking (abort) of inconsistencies
-			of the internal screen state. This is mosly
+			of the internal screen state. This is mostly
 			useful when running nvim inside a debugger (and
 			the test suite).
+	    nodelta	Send all internally redrawn cells to the UI, even if
+	                they are unchanged from the already displayed state.
 
 						*'redrawtime'* *'rdt'*
 'redrawtime' 'rdt'	number	(default 2000)
@@ -6916,7 +6918,6 @@ A jump table for the options with a short description can be found at |Q_op|.
 			global
 	The number of milliseconds to wait for each character sent to the
 	screen.  When positive, characters are sent to the UI one by one.
-	When negative, all redrawn characters cause a delay, even if the
+	See 'redrawdebug' for more options.  For debugging purposes.
-	character already was displayed by the UI.  For debugging purposes.
 
  vim:tw=78:ts=8:noet:ft=help:norl:

src/nvim/option.c

@@ -4306,6 +4306,10 @@ static char *set_num_option(int opt_idx, char_u *varp, long value,
     if (value < 0) {
       errmsg = e_positive;
     }
+  } else if (pp == &p_wd) {
+    if (value < 0) {
+      errmsg = e_positive;
+    }
   }
 
   // Don't change the value and return early if validation failed.

src/nvim/option_defs.h

@@ -518,11 +518,18 @@ EXTERN long p_pyx;              // 'pyxversion'
 EXTERN char_u *p_rdb;           // 'redrawdebug'
 EXTERN unsigned rdb_flags;
 # ifdef IN_OPTION_C
-static char *(p_rdb_values[]) = { "compositor", "nothrottle", "invalid", NULL };
+static char *(p_rdb_values[]) = {
+  "compositor",
+  "nothrottle",
+  "invalid",
+  "nodelta",
+  NULL
+};
 # endif
 # define RDB_COMPOSITOR         0x001
 # define RDB_NOTHROTTLE         0x002
 # define RDB_INVALID            0x004
+# define RDB_NODELTA            0x008
 
 EXTERN long p_rdt;              // 'redrawtime'
 EXTERN int p_remap;             // 'remap'

src/nvim/screen.c

@@ -577,7 +577,7 @@ void conceal_check_cursor_line(void)
 /// Whether cursorline is drawn in a special way
 ///
 /// If true, both old and new cursorline will need
-/// need to be redrawn when moving cursor within windows.
+/// to be redrawn when moving cursor within windows.
 /// TODO(bfredl): VIsual_active shouldn't be needed, but is used to fix a glitch
 ///               caused by scrolling.
 bool win_cursorline_standout(const win_T *wp)
@@ -2406,10 +2406,10 @@ win_line (
   filler_todo = filler_lines;
 
   // Cursor line highlighting for 'cursorline' in the current window.
-  if (wp->w_p_cul && lnum == wp->w_cursor.lnum) {
+  if (lnum == wp->w_cursor.lnum) {
     // Do not show the cursor line when Visual mode is active, because it's
     // not clear what is selected then.
-    if (!(wp == curwin && VIsual_active)) {
+    if (wp->w_p_cul && !(wp == curwin && VIsual_active)) {
       int cul_attr = win_hl_attr(wp, HLF_CUL);
       HlAttrs ae = syn_attr2entry(cul_attr);
 
@@ -4354,7 +4354,7 @@ static int grid_char_needs_redraw(ScreenGrid *grid, int off_from, int off_to,
                || (line_off2cells(linebuf_char, off_from, off_from + cols) > 1
                    && schar_cmp(linebuf_char[off_from + 1],
                                 grid->chars[off_to + 1])))
-              || p_wd < 0));
+              || rdb_flags & RDB_NODELTA));
 }
 
 /// Move one buffered line to the window grid, but only the characters that

test/functional/ui/syntax_conceal_spec.lua

@@ -1,6 +1,7 @@
 local helpers = require('test.functional.helpers')(after_each)
 local Screen = require('test.functional.ui.screen')
 local clear, feed, command = helpers.clear, helpers.feed, helpers.command
+local eq = helpers.eq
 local insert = helpers.insert
 
 describe('Screen', function()
@@ -870,4 +871,50 @@ describe('Screen', function()
       ]]}
     end)
   end)
+
+  it('redraws not too much with conceallevel=1', function()
+    command('set conceallevel=1')
+    command('set redrawdebug+=nodelta')
+
+    insert([[
+    aaa
+    bbb
+    ccc
+    ]])
+    screen:expect{grid=[[
+      aaa                                                  |
+      bbb                                                  |
+      ccc                                                  |
+      ^                                                     |
+      {0:~                                                    }|
+      {0:~                                                    }|
+      {0:~                                                    }|
+      {0:~                                                    }|
+      {0:~                                                    }|
+                                                           |
+    ]]}
+
+    -- XXX: hack to get notifications, and check only a single line is
+    --      updated.  Could use next_msg() also.
+    local orig_handle_grid_line = screen._handle_grid_line
+    local grid_lines = {}
+    function screen._handle_grid_line(self, grid, row, col, items)
+      table.insert(grid_lines, {row, col, items})
+      orig_handle_grid_line(self, grid, row, col, items)
+    end
+    feed('k')
+    screen:expect{grid=[[
+      aaa                                                  |
+      bbb                                                  |
+      ^ccc                                                  |
+                                                           |
+      {0:~                                                    }|
+      {0:~                                                    }|
+      {0:~                                                    }|
+      {0:~                                                    }|
+      {0:~                                                    }|
+                                                           |
+    ]]}
+    eq(grid_lines, {{2, 0, {{'c', 0, 3}}}})
+  end)
 end)