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

(cherry picked from commit ce0c0c31a0)

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
+               && (topline_conceal < wp->w_lines[0].wl_lnum
-                   || (wp->w_topline == 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;

test/functional/ui/decorations_spec.lua

@@ -2956,6 +2956,28 @@ describe('extmark decorations', function()
       {1:~                                                 }|*3
                                                         |
     ]])
+    -- No scrolling for concealed topline #33033
+    api.nvim_buf_clear_namespace(0, ns, 0, -1)
+    api.nvim_buf_set_extmark(0, ns, 1, 0, { virt_lines_above = true, virt_lines = { { { "virt_above 2" } } } })
+    api.nvim_buf_set_extmark(0, ns, 0, 0, { conceal_lines = "" })
+    feed('ggjj')
+    screen:expect([[
+      {2:    }virt_above 2                                  |
+      {2:  2 }    local text, hl_id_cell, count = unpack(ite|
+      {2:    }m)                                            |
+      {2:  3 }^    if hl_id_cell ~= nil then                 |
+      {2:  4 }        hl_id = hl_id_cell                    |
+      {2:  5 }conceal text                                  |
+      {2:  6 }    for _ = 1, (count or 1) do                |
+      {2:  7 }        local cell = line[colpos]             |
+      {2:  8 }        cell.text = text                      |
+      {2:  9 }        cell.hl_id = hl_id                    |
+      {2: 10 }        colpos = colpos+1                     |
+      {2: 11 }    end                                       |
+      {2: 12 }end                                           |
+      {1:~                                                 }|
+                                                        |
+    ]])
   end)
 end)