fix(decor): highlight without 'hl_eol' bleeds into wrap gaps #41160

Problem:
'linebreak' filler and 'breakindent'/'showbreak' padding are screen
cells with no buffer character behind them, yet a decoration draws over
them whether or not it asked to cover such cells. A highlight bounded to
its text then paints a tail out to the edge of the row, most visible on
inline code spans from plugins. That same highlight already leaves the
cells past the end of a line alone, so it treats identical cells two
different ways.

Solution:
Only a decoration with 'hl_eol' draws the gaps, which is what the flag
already means at the end of a line. A full-width background such as a
fenced code block sets it and still covers them. Classic :syntax has no
such flag and is unchanged.

test/functional/legacy/listlbr_spec.lua

@@ -330,22 +330,36 @@ describe('listlbr', function()
     ]])
   end)
 
-  it('extends a background highlight into the filler for the last word on a line', function()
+  it('extends a background highlight into the filler only with hl_eol', function()
     local screen = Screen.new(20, 5)
     screen:add_extra_attr_ids({ [100] = { background = Screen.colors.Red } })
     source([[
       set wrap linebreak
       call setline(1, 'word1.word2verylongwordthatpushesdown')
       highlight TestBg guibg=Red ctermbg=Red
+      let g:ns = nvim_create_namespace('')
     ]])
 
-    -- The pushed-down word is the last thing on the line, so nothing absorbs the width overflow: a
-    -- background highlight must still extend into the filler, since a solid color looks fine over
-    -- blank cells (unlike underline/strikethrough, see tests above).
-    command([[call nvim_buf_add_highlight(0, -1, 'TestBg', 0, 0, -1)]])
+    -- 'hl_eol' covers cells with no text behind them, so the filler is covered too.
+    command(
+      'call nvim_buf_set_extmark(0, g:ns, 0, 0, '
+        .. "{'end_row': 1, 'hl_group': 'TestBg', 'hl_eol': v:true})"
+    )
     screen:expect([[
       {100:^word1.              }|
       {100:word2verylongwordtha}|
+      {100:tpushesdown         }|
+      {1:~                   }|
+                          |
+    ]])
+
+    -- Without it the highlight decorates characters, so it stops where they do, just as an
+    -- underline already does (see tests above).
+    command('call nvim_buf_clear_namespace(0, -1, 0, -1)')
+    command([[call nvim_buf_add_highlight(0, -1, 'TestBg', 0, 0, -1)]])
+    screen:expect([[
+      {100:^word1.}              |
+      {100:word2verylongwordtha}|
       {100:tpushesdown}         |
       {1:~                   }|
                           |

test/functional/ui/decorations_spec.lua

@@ -767,7 +767,7 @@ describe('decorations providers', function()
     }
   end)
 
-  it("'hl_eol' and plain highlights extend into 'linebreak'/'breakindent'/'showbreak' gaps", function()
+  it("'hl_eol' extends into 'linebreak'/'breakindent'/'showbreak' gaps", function()
     command('set wrap linebreak breakindent breakindentopt=shift:2')
     api.nvim_buf_set_lines(0, 0, -1, false, {
       '  if vim.g.colors_name == "ferricferric" then',
@@ -794,31 +794,31 @@ describe('decorations providers', function()
                                               |
     ]])
 
-    -- No hl_eol here: these gaps have no real text, but the highlight is still active, not
-    -- ending, so it must show through regardless.
+    -- Without 'hl_eol' the highlight decorates characters, so it leaves the gaps alone, the same
+    -- way it already leaves the cells past the end of the line alone.
     api.nvim_buf_set_extmark(0, ns, 0, 0, { id = id, end_row = 1, hl_group = 'DiffAdd' })
     screen:expect([[
-      {100:^  if vim.g.colors_name ==               }|
-      {100:    }{101:>}{100:"ferricferric" then}                |
+      {100:^  if vim.g.colors_name == }              |
+          {1:>}{100:"ferricferric" then}                |
       {1:~                                       }|*5
                                               |
     ]])
   end)
 
   it('breakindent keeps the highlight after a real linebreak word-push', function()
-    -- The word pushed down by 'linebreak' resets decor_attr (it hides real word text, see listlbr_spec.lua), which must not then leak into
-    -- 'breakindent's own gap on the row the word lands on.
+    -- The word pushed down by 'linebreak' resets decor_attr (see listlbr_spec.lua), which must not
+    -- then leak into 'breakindent's own gap on the row the word lands on.
     screen:try_resize(30, 5)
     screen:add_extra_attr_ids({ [100] = { background = Screen.colors.LightBlue } })
     command('set wrap linebreak breakindent')
     local line = '      This is a long indented line of plain text that will wrap nicely.'
     api.nvim_buf_set_lines(0, 0, -1, false, { line })
     local ns = api.nvim_create_namespace('code_bg2')
-    api.nvim_buf_set_extmark(0, ns, 0, 0, { end_row = 0, end_col = #line, hl_group = 'DiffAdd' })
+    api.nvim_buf_set_extmark(0, ns, 0, 0, { end_row = 1, hl_group = 'DiffAdd', hl_eol = true })
     screen:expect([[
       {100:^      This is a long indented }|
       {100:      line of plain text that }|
-      {100:      will wrap nicely.}       |
+      {100:      will wrap nicely.       }|
       {1:~                             }|
                                     |
     ]])