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)
https://github.com/vim/vim/commit/841c225b9ef8c5bdf5e02968a0bd62521fff6ca8

1 files changed, 15 insertions, 6 deletions

diff --git a/src/nvim/diff.c b/src/nvim/diff.c
index 80b8920a7f..efbc2fd0db 100644
--- a/src/nvim/diff.c
+++ b/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) {
-    // The current window was set to used the maximum number of filler
-    // lines, may need to reduce them.
-    diff_set_topline(wp_other, curwin);
+  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);
+    }
   }
 }