fix(display): scroll logic does not take into account concealed topline (#33054)

1 files changed, 13 insertions, 3 deletions

diff --git a/src/nvim/drawscreen.c b/src/nvim/drawscreen.c
index 4c1b756ea1..2905d51657 100644
--- a/src/nvim/drawscreen.c
+++ b/src/nvim/drawscreen.c
@@ -1700,6 +1700,16 @@ static void win_update(win_T *wp)
     }
   }
 
+  // Below logic compares wp->w_topline against wp->w_lines[0].wl_lnum,
+  // which may point to a line below wp->w_topline if it is concealed;
+  // incurring scrolling even though wp->w_topline is still the same.
+  // Compare against an adjusted topline instead:
+  linenr_T topline_conceal = wp->w_topline;
+  while (decor_conceal_line(wp, topline_conceal - 1, false)) {
+    topline_conceal++;
+    hasFolding(wp, topline_conceal, NULL, &topline_conceal);
+  }
+
   // If there are no changes on the screen that require a complete redraw,
   // handle three cases:
   // 1: we are off the top of the screen by a few lines: scroll down
@@ -1712,12 +1722,12 @@ static void win_update(win_T *wp)
     if (mod_top != 0
         && wp->w_topline == mod_top
         && (!wp->w_lines[0].wl_valid
-            || wp->w_topline == wp->w_lines[0].wl_lnum)) {
+            || topline_conceal == wp->w_lines[0].wl_lnum)) {
       // w_topline is the first changed line and window is not scrolled,
       // the scrolling from changed lines will be done further down.
     } else if (wp->w_lines[0].wl_valid
-               && (wp->w_topline < wp->w_lines[0].wl_lnum
-                   || (wp->w_topline == wp->w_lines[0].wl_lnum
+               && (topline_conceal < wp->w_lines[0].wl_lnum
+                   || (topline_conceal == wp->w_lines[0].wl_lnum
                        && wp->w_topfill > wp->w_old_topfill))) {
       // New topline is above old topline: May scroll down.
       int j;