From 749126de087db45bb0ee31c6785dbdb1d30336c9 Mon Sep 17 00:00:00 2001
From: glepnir <glephunter@gmail.com>
Date: Tue, 16 Jun 2026 19:39:32 +0800
Subject: [PATCH] fix(highlight): keep guisp underline color in float with
 'winblend' #40283

Problem:
A colored (guisp) underline showing through a 'winblend' float
lost its special color and followed the foreground instead.

Solution:
In the blend-through case, blend the underline's special color
only when the cell below sets sp explicitly; otherwise clear it.

(cherry picked from commit 823517ad1d5df03914c55ec7ca5daa504734530d)
---
 src/nvim/highlight.c              |  4 ++--
 test/functional/ui/float_spec.lua | 20 +++++++++++---------
 2 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/src/nvim/highlight.c b/src/nvim/highlight.c
index 99f0d8d56c..7f4fdb5df6 100644
--- a/src/nvim/highlight.c
+++ b/src/nvim/highlight.c
@@ -737,8 +737,8 @@ int hl_blend_attrs(int back_attr, int front_attr, bool *through)
   if (*through) {
     cattrs = battrs;
     cattrs.rgb_fg_color = rgb_blend(ratio, battrs.rgb_fg_color, fattrs.rgb_bg_color);
-    // Only apply special colors when the foreground attribute has an underline style.
-    if (fattrs_raw.rgb_ae_attr & HL_UNDERLINE_MASK) {
+    // Blend the special color only when the cell below sets one explicitly, else clear it.
+    if ((cattrs.rgb_ae_attr & HL_UNDERLINE_MASK) && battrs_raw.rgb_sp_color != -1) {
       cattrs.rgb_sp_color = rgb_blend(ratio, battrs.rgb_sp_color, fattrs.rgb_bg_color);
     } else {
       cattrs.rgb_sp_color = -1;
diff --git a/test/functional/ui/float_spec.lua b/test/functional/ui/float_spec.lua
index 1d467d17ec..32915086a7 100644
--- a/test/functional/ui/float_spec.lua
+++ b/test/functional/ui/float_spec.lua
@@ -8933,6 +8933,8 @@ describe('float window', function()
         [32] = { foreground = Screen.colors.Blue1, blend = 100, bold = true },
         [33] = { foreground = Screen.colors.Gray0, underline = true },
         [34] = { underline = true },
+        [35] = { foreground = Screen.colors.Black, underline = true, special = Screen.colors.Red },
+        [36] = { special = Screen.colors.Red, underline = true },
       })
       insert([[
         Lorem ipsum dolor sit amet, consectetur
@@ -9292,12 +9294,12 @@ describe('float window', function()
         ]])
       end
 
-      -- winblend highlight with underline (but without guisp) in a floatwin. #14453
-      command('fclose | hi TestUnderLine gui=underline')
+      -- winblend underline: without guisp follows the fg (#14453), with guisp keeps its sp (#34614).
+      command('fclose | hi TestUnderLine gui=underline | hi TestUnderLineSp gui=underline guisp=Red')
       api.nvim_buf_add_highlight(curbufnr, -1, 'TestUnderLine', 3, 0, -1)
-      api.nvim_buf_add_highlight(curbufnr, -1, 'TestUnderLine', 4, 0, -1)
-      api.nvim_buf_set_lines(buf, 0, -1, false, {})
-      api.nvim_open_win(buf, false, { relative = 'win', row = 0, col = 0, width = 50, height = 1 })
+      api.nvim_buf_add_highlight(curbufnr, -1, 'TestUnderLineSp', 4, 0, -1)
+      api.nvim_buf_set_lines(buf, 0, -1, false, { '', '' })
+      api.nvim_open_win(buf, false, { relative = 'win', row = 0, col = 0, width = 50, height = 2 })
       if multigrid then
         screen:expect({
           grid = [[
@@ -9306,7 +9308,7 @@ describe('float window', function()
             [3:--------------------------------------------------]|
           ## grid 2
             {34:Ut enim ad minim veniam, quis nostrud}             |
-            {34:exercitation ullamco laboris nisi ut aliquip ex}   |
+            {36: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   |
@@ -9316,13 +9318,13 @@ describe('float window', function()
           ## grid 3
                                                               |
           ## grid 5
-            {17:                                                  }|
+            {17:                                                  }|*2
           ]],
           win_pos = { [2] = { height = 8, startcol = 0, startrow = 0, width = 50, win = 1000 } },
           float_pos = { [5] = { 1002, 'NW', 2, 0, 0, true, 50, 1, 0, 0 } },
           win_viewport = {
             [2] = { win = 1000, topline = 3, botline = 11, curline = 9, curcol = 0, linecount = 11, sum_scroll_delta = 3 },
-            [5] = { win = 1002, topline = 0, botline = 1, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0 },
+            [5] = { win = 1002, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0 },
           },
           win_viewport_margins = {
             [2] = { bottom = 0, left = 0, right = 0, top = 0, win = 1000 },
@@ -9332,7 +9334,7 @@ describe('float window', function()
       else
         screen:expect([[
           {33:Ut enim ad minim veniam, quis nostrud}{26:             }|
-          {34:exercitation ullamco laboris nisi ut aliquip ex}   |
+          {35:exercitation ullamco laboris nisi ut aliquip ex}{26:   }|
           ea commodo consequat. Duis aute irure dolor in    |
           reprehenderit in voluptate velit esse cillum      |
           dolore eu fugiat nulla pariatur. Excepteur sint   |