diff/highlight: Show underline for low-priority CursorLine

3 files changed, 57 insertions, 3 deletions

diff --git a/src/nvim/highlight.c b/src/nvim/highlight.c
index ae2b90d8a1..89a41e73de 100644
--- a/src/nvim/highlight.c
+++ b/src/nvim/highlight.c
@@ -177,6 +177,24 @@ void update_window_hl(win_T *wp, bool invalid)
   }
 }
 
+/// Gets HL_UNDERLINE highlight.
+int hl_get_underline(void)
+{
+  return get_attr_entry((HlEntry){
+      .attr = (HlAttrs){
+          .cterm_ae_attr = (int16_t)HL_UNDERLINE,
+          .cterm_fg_color = 0,
+          .cterm_bg_color = 0,
+          .rgb_ae_attr = (int16_t)HL_UNDERLINE,
+          .rgb_fg_color = 0,
+          .rgb_bg_color = 0,
+      },
+      .kind = kHlUI,
+      .id1 = 0,
+      .id2 = 0,
+  });
+}
+
 /// Get attribute code for forwarded :terminal highlights.
 int hl_get_term_attr(HlAttrs *aep)
 {
diff --git a/src/nvim/screen.c b/src/nvim/screen.c
index 2a1dae1767..eb24e2af1c 100644
--- a/src/nvim/screen.c
+++ b/src/nvim/screen.c
@@ -3052,9 +3052,13 @@ win_line (
           diff_hlf = HLF_CHD;                   // changed line
         }
         line_attr = win_hl_attr(wp, diff_hlf);
-        // Overlay CursorLine onto diff highlight, unless it's low-priority.
-        if (!line_attr_lowprio && wp->w_p_cul && lnum == wp->w_cursor.lnum) {
-          line_attr = hl_combine_attr(line_attr, win_hl_attr(wp, HLF_CUL));
+        // Overlay CursorLine onto diff-mode highlight.
+        if (wp->w_p_cul && lnum == wp->w_cursor.lnum) {
+          line_attr = 0 != line_attr_lowprio  // Low-priority CursorLine
+            ? hl_combine_attr(hl_combine_attr(win_hl_attr(wp, HLF_CUL),
+                                              line_attr),
+                              hl_get_underline())
+            : hl_combine_attr(line_attr, win_hl_attr(wp, HLF_CUL));
         }
       }
 
diff --git a/test/functional/ui/highlight_spec.lua b/test/functional/ui/highlight_spec.lua
index 55fc343e4c..dffe5b5197 100644
--- a/test/functional/ui/highlight_spec.lua
+++ b/test/functional/ui/highlight_spec.lua
@@ -768,6 +768,38 @@ describe('CursorLine highlight', function()
       {4:[No Name] [+]             }{9:[No Name] [+]           }|
                                                         |
     ]])
+
+    -- CursorLine with fg=NONE is "low-priority".
+    -- Rendered as underline in a diff-line. #9028
+    command('hi CursorLine ctermbg=red ctermfg=NONE guibg=red guifg=NONE')
+    feed('kkkk')
+    screen:expect([[
+      {1:  }line 1 some text       {4:│}{1:  }line 1 some text      |
+      {1:  }{11:line 2 mo}{12:Re text!}{11:      }{4:│}{1:  }{11:^line 2 mo}{12:re text}{11:      }|
+      {1:  }{5:extra line!            }{4:│}{1:  }{6:----------------------}|
+      {1:  }extra line!            {4:│}{1:  }extra line!           |
+      {1:  }extra line!            {4:│}{1:  }extra line!           |
+      {1:  }last line ...          {4:│}{1:  }last line ...         |
+      {1:  }                       {4:│}{1:  }                      |
+      {1:  }{8:~                      }{4:│}{1:  }{8:~                     }|
+      {1:  }{8:~                      }{4:│}{1:  }{8:~                     }|
+      {1:  }{8:~                      }{4:│}{1:  }{8:~                     }|
+      {4:[No Name] [+]             }{9:[No Name] [+]           }|
+                                                        |
+    ]], {
+      [1] = {foreground = Screen.colors.DarkBlue, background = Screen.colors.WebGray},
+      [2] = {bold = true, background = Screen.colors.Red},
+      [3] = {background = Screen.colors.LightMagenta},
+      [4] = {reverse = true},
+      [5] = {background = Screen.colors.LightBlue},
+      [6] = {background = Screen.colors.LightCyan1, bold = true, foreground = Screen.colors.Blue1},
+      [7] = {foreground = Screen.colors.Grey100, background = Screen.colors.Red},
+      [8] = {bold = true, foreground = Screen.colors.Blue1},
+      [9] = {bold = true, reverse = true},
+      [10] = {bold = true},
+      [11] = {special = Screen.colors.Grey0, underline = true, foreground = Screen.colors.Grey0, background = Screen.colors.Grey0},
+      [12] = {bold = true, special = Screen.colors.Grey0, background = Screen.colors.Grey0, foreground = Screen.colors.Grey0, underline = true},
+    })
   end)
 end)