fix(decor): extmark highlight not applied (#33858)

Problem: If the only highlight present in the buffer is an extmark, and its end
position is outside the screen, redraws that start on lines after the
first line of the mark will consider the buffer as not having any highlights,
and skip drawing the mark's highlight.
Solution: Check the updated number of decor ranges.

src/nvim/decoration.c

@@ -493,7 +493,7 @@ bool decor_redraw_start(win_T *wp, int top_row, DecorState *state)
   return true;  // TODO(bfredl): check if available in the region
 }
 
-bool decor_redraw_line(win_T *wp, int row, DecorState *state)
+static void decor_state_pack(DecorState *state)
 {
   int count = (int)kv_size(state->ranges_i);
   int const cur_end = state->current_end;
@@ -513,6 +513,11 @@ bool decor_redraw_line(win_T *wp, int row, DecorState *state)
 
   kv_size(state->ranges_i) = (size_t)count;
   state->future_begin = fut_beg;
+}
+
+bool decor_redraw_line(win_T *wp, int row, DecorState *state)
+{
+  decor_state_pack(state);
 
   if (state->row == -1) {
     decor_redraw_start(wp, row, state);
@@ -525,7 +530,7 @@ bool decor_redraw_line(win_T *wp, int row, DecorState *state)
   state->col_until = -1;
   state->eol_col = -1;
 
-  if (cur_end != 0 || fut_beg != count) {
+  if (state->current_end != 0 || state->future_begin != (int)kv_size(state->ranges_i)) {
     return true;
   }
 

test/functional/ui/decorations_spec.lua

@@ -3402,6 +3402,28 @@ describe('extmark decorations', function()
                                                         |
     ]])
   end)
+
+  it('redraws extmark that starts and ends outisde the screen', function()
+    local lines = vim.split(('1'):rep(20), '', { plain = true })
+    api.nvim_buf_set_lines(0, 0, -1, true, lines)
+    api.nvim_buf_set_extmark(0, ns, 0, 0, { hl_group = 'ErrorMsg', end_row = 19, end_col = 0 })
+    screen:expect({
+      grid = [[
+        {4:^1}                                                 |
+        {4:1}                                                 |*13
+                                                          |
+      ]]
+    })
+    feed('<C-e>')
+    -- Newly visible line should also have the highlight.
+    screen:expect({
+      grid = [[
+        {4:^1}                                                 |
+        {4:1}                                                 |*13
+                                                          |
+      ]]
+    })
+  end)
 end)
 
 describe('decorations: inline virtual text', function()