From 52669b5c690e81a33df5a97f1f93682f63dff875 Mon Sep 17 00:00:00 2001
From: Jaehwang Jung <tomtomjhj@gmail.com>
Date: Sat, 12 Aug 2023 17:17:10 +0900
Subject: [PATCH] fix(diff): filler lines for hunks bigger than linematch limit

Apply linematch filler computation only if the hunk is actually
linematched.

Fixes #24580

(cherry picked from commit ed1da3ac24bfe8f5a499f837feaa7506778e8969)
---
 src/nvim/diff.c                       |  2 +-
 test/functional/ui/linematch_spec.lua | 44 +++++++++++++++++++++++++++
 2 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/src/nvim/diff.c b/src/nvim/diff.c
index 52c5732f23..5856bb9af8 100644
--- a/src/nvim/diff.c
+++ b/src/nvim/diff.c
@@ -2383,7 +2383,7 @@ void diff_set_topline(win_T *fromwin, win_T *towin)
     towin->w_topline = lnum + (dp->df_lnum[toidx] - dp->df_lnum[fromidx]);
 
     if (lnum >= dp->df_lnum[fromidx]) {
-      if (diff_flags & DIFF_LINEMATCH) {
+      if (dp->is_linematched) {
         calculate_topfill_and_topline(fromidx, toidx, fromwin->w_topline,
                                       fromwin->w_topfill, &towin->w_topfill, &towin->w_topline);
       } else {
diff --git a/test/functional/ui/linematch_spec.lua b/test/functional/ui/linematch_spec.lua
index 697677aa67..1a5ec36a28 100644
--- a/test/functional/ui/linematch_spec.lua
+++ b/test/functional/ui/linematch_spec.lua
@@ -992,4 +992,48 @@ describe('regressions', function()
     helpers.curbufmeths.set_lines(0, -1, false, { string.rep('a', 1010)..'world' })
     helpers.exec 'windo diffthis'
   end)
+
+  it("properly computes filler lines for hunks bigger than linematch limit", function()
+    clear()
+    feed(':set diffopt+=linematch:10<cr>')
+    screen = Screen.new(100, 20)
+    screen:attach()
+    local lines = {}
+    for i = 0, 29 do
+      lines[#lines + 1] = tostring(i)
+    end
+    helpers.curbufmeths.set_lines(0, -1, false, lines)
+    helpers.exec 'vnew'
+    helpers.curbufmeths.set_lines(0, -1, false, { '00', '29' })
+    helpers.exec 'windo diffthis'
+    feed('<C-e>')
+    screen:expect{grid=[[
+      {1:  }{2:------------------------------------------------}│{1:  }{3:^1                                              }|
+      {1:  }{2:------------------------------------------------}│{1:  }{3:2                                              }|
+      {1:  }{2:------------------------------------------------}│{1:  }{3:3                                              }|
+      {1:  }{2:------------------------------------------------}│{1:  }{3:4                                              }|
+      {1:  }{2:------------------------------------------------}│{1:  }{3:5                                              }|
+      {1:  }{2:------------------------------------------------}│{1:  }{3:6                                              }|
+      {1:  }{2:------------------------------------------------}│{1:  }{3:7                                              }|
+      {1:  }{2:------------------------------------------------}│{1:  }{3:8                                              }|
+      {1:  }{2:------------------------------------------------}│{1:  }{3:9                                              }|
+      {1:  }{2:------------------------------------------------}│{1:  }{3:10                                             }|
+      {1:  }{2:------------------------------------------------}│{1:  }{3:11                                             }|
+      {1:  }{2:------------------------------------------------}│{1:  }{3:12                                             }|
+      {1:  }{2:------------------------------------------------}│{1:  }{3:13                                             }|
+      {1:  }{2:------------------------------------------------}│{1:  }{3:14                                             }|
+      {1:  }{2:------------------------------------------------}│{1:  }{3:15                                             }|
+      {1:  }{2:------------------------------------------------}│{1:  }{3:16                                             }|
+      {1:  }{2:------------------------------------------------}│{1:  }{3:17                                             }|
+      {1:  }29                                              │{1:  }{3:18                                             }|
+      {4:[No Name] [+]                                      }{5:[No Name] [+]                                    }|
+                                                                                                          |
+    ]], attr_ids={
+      [1] = {foreground = Screen.colors.DarkBlue, background = Screen.colors.Grey};
+      [2] = {bold = true, background = Screen.colors.LightCyan, foreground = Screen.colors.Blue1};
+      [3] = {background = Screen.colors.LightBlue};
+      [4] = {reverse = true};
+      [5] = {reverse = true, bold = true};
+    }}
+  end)
 end)