vim-patch:8.2.3556: filler lines are incorrect for other window in diff mode (#16164)

Problem:    Filler lines are incorrect for other window in diff mode after
            making a change.
Solution:   Copy filler lines from the current window. (closes vim/vim#8809)
841c225b9e

src/nvim/diff.c

@@ -638,7 +638,8 @@ static int diff_check_sanity(tabpage_T *tp, diff_T *dp)
 void diff_redraw(bool dofold)
 {
   win_T *wp_other = NULL;
-  bool used_max_fill = false;
+  bool used_max_fill_other = false;
+  bool used_max_fill_curwin = false;
 
   need_diff_redraw = false;
   FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
@@ -663,16 +664,24 @@ void diff_redraw(bool dofold)
       } else if ((n > 0) && (n > wp->w_topfill)) {
         wp->w_topfill = n;
         if (wp == curwin) {
-          used_max_fill = true;
+          used_max_fill_curwin = true;
+        } else if (wp_other != NULL) {
+          used_max_fill_other = true;
         }
       }
       check_topfill(wp, false);
     }
   }
-  if (wp_other != NULL && used_max_fill && curwin->w_p_scb) {
+  if (wp_other != NULL && curwin->w_p_scb) {
+    if (used_max_fill_curwin) {
       // The current window was set to used the maximum number of filler
       // lines, may need to reduce them.
       diff_set_topline(wp_other, curwin);
+    } else if (used_max_fill_other) {
+      // The other window was set to used the maximum number of filler
+      // lines, may need to reduce them.
+      diff_set_topline(curwin, wp_other);
+    }
   }
 }
 

src/nvim/testdir/test_diffmode.vim

@@ -984,6 +984,9 @@ func Test_diff_with_scroll_and_change()
   call term_sendkeys(buf, "ax\<Esc>")
   call VerifyScreenDump(buf, 'Test_diff_scroll_change_02', {})
 
+  call term_sendkeys(buf, "\<C-W>lay\<Esc>")
+  call VerifyScreenDump(buf, 'Test_diff_scroll_change_03', {})
+
   " clean up
   call StopVimInTerminal(buf)
   call delete('Xtest_scroll_change')

test/functional/ui/diff_spec.lua

@@ -1196,4 +1196,27 @@ it('Align the filler lines when changing text in diff mode', function()
     {8:[No Name] [+]        }{3:[No Name] [+]      }|
                                             |
   ]]}
+  feed('<C-W>lay<Esc>')
+  screen:expect{grid=[[
+    {1:  }{2:-----------------}{3:│}{1:  }{4:6                 }|
+    {1:  }{2:-----------------}{3:│}{1:  }{4:7                 }|
+    {1:  }{2:-----------------}{3:│}{1:  }{4:8                 }|
+    {1:  }9                {3:│}{1:  }9                 |
+    {1:  }10               {3:│}{1:  }10                |
+    {1:  }11               {3:│}{1:  }11                |
+    {1:  }12               {3:│}{1:  }12                |
+    {1:  }13               {3:│}{1:  }13                |
+    {1:  }14               {3:│}{1:  }14                |
+    {1:  }{5:15}{6:x}{5:              }{3:│}{1:  }{5:15}{6:^y}{5:               }|
+    {7:~                  }{3:│}{7:~                   }|
+    {7:~                  }{3:│}{7:~                   }|
+    {7:~                  }{3:│}{7:~                   }|
+    {7:~                  }{3:│}{7:~                   }|
+    {7:~                  }{3:│}{7:~                   }|
+    {7:~                  }{3:│}{7:~                   }|
+    {7:~                  }{3:│}{7:~                   }|
+    {7:~                  }{3:│}{7:~                   }|
+    {3:[No Name] [+]       }{8:[No Name] [+]       }|
+                                            |
+  ]]}
 end)