fix(winblend): treat braille blank (\u2800) as whitespace #32741

Problem:
'winblend' does not display text behind blank unicode characters other than 0x20 ascii space.

Solution:
In ui_compositor.c -> compose_line() check for either 0x20 or 0x80A0E2 (utf8 \u2800).

src/nvim/ui_compositor.c

@@ -422,10 +422,12 @@ static void compose_line(Integer row, Integer startcol, Integer endcol, LineFlag
       for (int i = col - (int)startcol; i < until - startcol; i += width) {
         width = 1;
         // negative space
-        bool thru = linebuf[i] == schar_from_ascii(' ') && bg_line[i] != NUL;
+        bool thru = (linebuf[i] == schar_from_ascii(' ')
+                     || linebuf[i] == schar_from_char(L'\u2800')) && bg_line[i] != NUL;
         if (i + 1 < endcol - startcol && bg_line[i + 1] == NUL) {
           width = 2;
-          thru &= linebuf[i + 1] == schar_from_ascii(' ');
+          thru &= (linebuf[i + 1] == schar_from_ascii(' ')
+                   || linebuf[i + 1] == schar_from_char(L'\u2800'));
         }
         attrbuf[i] = (sattr_T)hl_blend_attrs(bg_attrs[i], attrbuf[i], &thru);
         if (width == 2) {

test/functional/ui/float_spec.lua

@@ -7947,7 +7947,8 @@ describe('float window', function()
         qui officia deserunt mollit anim id est
         laborum.]])
       local buf = api.nvim_create_buf(false,false)
-      api.nvim_buf_set_lines(buf, 0, -1, true, {"test", "", "popup    text"})
+      local test_data = {"test", "", "popup    text"}
+      api.nvim_buf_set_lines(buf, 0, -1, true, test_data)
       local win = api.nvim_open_win(buf, false, {relative='editor', width=15, height=3, row=2, col=5})
       if multigrid then
         screen:expect{grid=[[
@@ -8020,6 +8021,45 @@ describe('float window', function()
         ]])
       end
 
+      -- Test for \u2800 (braille blank unicode character)
+      local braille_blank = "\226\160\128"
+      api.nvim_buf_set_lines(buf, 0, -1, true, {"test" .. braille_blank, "", "popup"..braille_blank.."   text"})
+      if multigrid then
+        screen:expect{grid=[[
+        ## grid 1
+          [2:--------------------------------------------------]|*8
+          [3:--------------------------------------------------]|
+        ## grid 2
+          Ut enim ad minim veniam, quis nostrud             |
+          exercitation ullamco laboris nisi ut aliquip ex   |
+          ea commodo consequat. Duis aute irure dolor in    |
+          reprehenderit in voluptate velit esse cillum      |
+          dolore eu fugiat nulla pariatur. Excepteur sint   |
+          occaecat cupidatat non proident, sunt in culpa    |
+          qui officia deserunt mollit anim id est           |
+          laborum^.                                          |
+        ## grid 3
+                                                            |
+        ## grid 4
+          {9:test]] .. braille_blank .. [[          }|
+          {9:               }|
+          {9:popup]] .. braille_blank .. [[   text  }|
+        ]], float_pos={[4] = {1001, "NW", 1, 2, 5, true, 50, 1, 2, 5}}, unchanged=true}
+      else
+        screen:expect([[
+          Ut enim ad minim veniam, quis nostrud             |
+          exercitation ullamco laboris nisi ut aliquip ex   |
+          ea co{2:test}{3:o consequat}. Duis aute irure dolor in    |
+          repre{3:henderit in vol}uptate velit esse cillum      |
+          dolor{2:popup}{3:fugi}{2:text}{3:ul}la pariatur. Excepteur sint   |
+          occaecat cupidatat non proident, sunt in culpa    |
+          qui officia deserunt mollit anim id est           |
+          laborum^.                                          |
+                                                            |
+        ]])
+      end
+      api.nvim_buf_set_lines(buf, 0, -1, true, test_data)
+
       -- Check that 'winblend' works with NormalNC highlight
       api.nvim_set_option_value('winhighlight', 'NormalNC:Visual', {win = win})
       if multigrid then