vim-patch:9.1.0822: topline might be changed in diff mode unexpectedly (#30988)

Problem:  topline might be changed in diff mode unexpectedly
          (Jaehwang Jung)
Solution: do not re-calculate topline, when using line() func
          in diff mode.

fixes: vim/vim#15812
closes: vim/vim#15950

https://github.com/vim/vim/commit/05a40e07c2f0e41b708c4c75a6aa7d0e7f6201a3

Co-authored-by: Christian Brabandt <cb@256bit.org>

3 files changed, 141 insertions, 0 deletions

diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index e8b9288717..8f676d7906 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -4351,9 +4351,17 @@ static void f_line(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
     if (wp != NULL && tp != NULL) {
       switchwin_T switchwin;
       if (switch_win_noblock(&switchwin, wp, tp, true) == OK) {
+        // in diff mode, prevent that the window scrolls
+        // and keep the topline
+        if (curwin->w_p_diff && switchwin.sw_curwin->w_p_diff) {
+          skip_update_topline = true;
+        }
         check_cursor(curwin);
         fp = var2fpos(&argvars[0], true, &fnum, false);
       }
+      if (curwin->w_p_diff && switchwin.sw_curwin->w_p_diff) {
+        skip_update_topline = false;
+      }
       restore_win_noblock(&switchwin, true);
     }
   } else {
diff --git a/test/functional/ui/diff_spec.lua b/test/functional/ui/diff_spec.lua
index d6a04f90f6..8b965e8a97 100644
--- a/test/functional/ui/diff_spec.lua
+++ b/test/functional/ui/diff_spec.lua
@@ -2054,3 +2054,109 @@ it('diff mode overlapped diff blocks will be merged', function()
                                        |
   ]])
 end)
+
+-- oldtest: Test_diff_topline_noscroll()
+it('diff mode does not scroll with line("w0")', function()
+  local screen = Screen.new(45, 20)
+  screen:attach()
+  exec([[
+    set scrolloff=5
+    call setline(1, range(1,60))
+    vnew
+    call setline(1, range(1,10) + range(50,60))
+    windo diffthis
+    norm! G
+    exe "norm! 30\<C-y>"
+  ]])
+  screen:expect([[
+    {7:  }9                   │{7:  }9                   |
+    {7:  }10                  │{7:  }10                  |
+    {7:  }{23:--------------------}│{7:  }{22:11                  }|
+    {7:  }{23:--------------------}│{7:  }{22:12                  }|
+    {7:  }{23:--------------------}│{7:  }{22:13                  }|
+    {7:  }{23:--------------------}│{7:  }{22:14                  }|
+    {7:  }{23:--------------------}│{7:  }{22:15                  }|
+    {7:  }{23:--------------------}│{7:  }{22:16                  }|
+    {7:  }{23:--------------------}│{7:  }{22:17                  }|
+    {7:  }{23:--------------------}│{7:  }{22:18                  }|
+    {7:  }{23:--------------------}│{7:  }{22:19                  }|
+    {7:  }{23:--------------------}│{7:  }{22:20                  }|
+    {7:  }{23:--------------------}│{7:  }{22:^21                  }|
+    {7:  }{23:--------------------}│{7:  }{22:22                  }|
+    {7:  }{23:--------------------}│{7:  }{22:23                  }|
+    {7:  }{23:--------------------}│{7:  }{22:24                  }|
+    {7:  }{23:--------------------}│{7:  }{22:25                  }|
+    {7:  }{23:--------------------}│{7:  }{22:26                  }|
+    {2:[No Name] [+]          }{3:[No Name] [+]         }|
+                                                 |
+  ]])
+  command([[echo line('w0', 1001)]])
+  screen:expect([[
+    {7:  }9                   │{7:  }9                   |
+    {7:  }10                  │{7:  }10                  |
+    {7:  }{23:--------------------}│{7:  }{22:11                  }|
+    {7:  }{23:--------------------}│{7:  }{22:12                  }|
+    {7:  }{23:--------------------}│{7:  }{22:13                  }|
+    {7:  }{23:--------------------}│{7:  }{22:14                  }|
+    {7:  }{23:--------------------}│{7:  }{22:15                  }|
+    {7:  }{23:--------------------}│{7:  }{22:16                  }|
+    {7:  }{23:--------------------}│{7:  }{22:17                  }|
+    {7:  }{23:--------------------}│{7:  }{22:18                  }|
+    {7:  }{23:--------------------}│{7:  }{22:19                  }|
+    {7:  }{23:--------------------}│{7:  }{22:20                  }|
+    {7:  }{23:--------------------}│{7:  }{22:^21                  }|
+    {7:  }{23:--------------------}│{7:  }{22:22                  }|
+    {7:  }{23:--------------------}│{7:  }{22:23                  }|
+    {7:  }{23:--------------------}│{7:  }{22:24                  }|
+    {7:  }{23:--------------------}│{7:  }{22:25                  }|
+    {7:  }{23:--------------------}│{7:  }{22:26                  }|
+    {2:[No Name] [+]          }{3:[No Name] [+]         }|
+    9                                            |
+  ]])
+  feed('<C-W>p')
+  screen:expect([[
+    {7:  }{23:--------------------}│{7:  }{22:39                  }|
+    {7:  }{23:--------------------}│{7:  }{22:40                  }|
+    {7:  }{23:--------------------}│{7:  }{22:41                  }|
+    {7:  }{23:--------------------}│{7:  }{22:42                  }|
+    {7:  }{23:--------------------}│{7:  }{22:43                  }|
+    {7:  }{23:--------------------}│{7:  }{22:44                  }|
+    {7:  }{23:--------------------}│{7:  }{22:45                  }|
+    {7:  }{23:--------------------}│{7:  }{22:46                  }|
+    {7:  }{23:--------------------}│{7:  }{22:47                  }|
+    {7:  }{23:--------------------}│{7:  }{22:48                  }|
+    {7:  }{23:--------------------}│{7:  }{22:49                  }|
+    {7:  }^50                  │{7:  }50                  |
+    {7:  }51                  │{7:  }51                  |
+    {7:  }52                  │{7:  }52                  |
+    {7:  }53                  │{7:  }53                  |
+    {7:  }54                  │{7:  }54                  |
+    {7:  }55                  │{7:  }55                  |
+    {7:+ }{13:+--  5 lines: 56····}│{7:+ }{13:+--  5 lines: 56····}|
+    {3:[No Name] [+]          }{2:[No Name] [+]         }|
+    9                                            |
+  ]])
+  feed('<C-W>p')
+  screen:expect([[
+    {7:  }{23:--------------------}│{7:  }{22:39                  }|
+    {7:  }{23:--------------------}│{7:  }{22:40                  }|
+    {7:  }{23:--------------------}│{7:  }{22:41                  }|
+    {7:  }{23:--------------------}│{7:  }{22:42                  }|
+    {7:  }{23:--------------------}│{7:  }{22:43                  }|
+    {7:  }{23:--------------------}│{7:  }{22:^44                  }|
+    {7:  }{23:--------------------}│{7:  }{22:45                  }|
+    {7:  }{23:--------------------}│{7:  }{22:46                  }|
+    {7:  }{23:--------------------}│{7:  }{22:47                  }|
+    {7:  }{23:--------------------}│{7:  }{22:48                  }|
+    {7:  }{23:--------------------}│{7:  }{22:49                  }|
+    {7:  }50                  │{7:  }50                  |
+    {7:  }51                  │{7:  }51                  |
+    {7:  }52                  │{7:  }52                  |
+    {7:  }53                  │{7:  }53                  |
+    {7:  }54                  │{7:  }54                  |
+    {7:  }55                  │{7:  }55                  |
+    {7:+ }{13:+--  5 lines: 56····}│{7:+ }{13:+--  5 lines: 56····}|
+    {2:[No Name] [+]          }{3:[No Name] [+]         }|
+    9                                            |
+  ]])
+end)
diff --git a/test/old/testdir/test_diffmode.vim b/test/old/testdir/test_diffmode.vim
index a448ed9b7f..a97006a7cc 100644
--- a/test/old/testdir/test_diffmode.vim
+++ b/test/old/testdir/test_diffmode.vim
@@ -2007,4 +2007,31 @@ func Test_diff_overlapped_diff_blocks_will_be_merged()
   call StopVimInTerminal(buf)
 endfunc
 
+" switching windows in diff mode caused an unneccessary scroll
+func Test_diff_topline_noscroll()
+  CheckScreendump
+
+  let content =<< trim END
+    call setline(1, range(1,60))
+    vnew
+    call setline(1, range(1,10) + range(50,60))
+    windo diffthis
+    norm! G
+    exe "norm! 30\<C-y>"
+  END
+  call writefile(content, 'Xcontent', 'D')
+  let buf = RunVimInTerminal('-S Xcontent', {'rows': 20})
+  call VerifyScreenDump(buf, 'Test_diff_topline_1', {})
+  call term_sendkeys(buf, ":echo line('w0', 1001)\<cr>")
+  call term_wait(buf)
+  call VerifyScreenDump(buf, 'Test_diff_topline_2', {})
+  call term_sendkeys(buf, "\<C-W>p")
+  call term_wait(buf)
+  call VerifyScreenDump(buf, 'Test_diff_topline_3', {})
+  call term_sendkeys(buf, "\<C-W>p")
+  call term_wait(buf)
+  call VerifyScreenDump(buf, 'Test_diff_topline_4', {})
+  call StopVimInTerminal(buf)
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab