fix(drawline): hang while redrawing diff filler above fold #39219

Problem:
win_line() falls into infinite loop when a diff window has top filler
above its first visible buffer line, that first visible buffer line is a
closed fold, and the folded line uses normal non-empty foldtext.

Solution:
Allow flushing pending diff filler rows even when the underlying buffer
line is folded with foldtext.

AI-assisted: Codex

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
(cherry picked from commit f2cc0a249d)

src/nvim/drawline.c

@@ -3105,7 +3105,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, b
 end_check:
     // At end of screen line and there is more to come: Display the line
     // so far.  If there is no more to display it is caught above.
-    if (wlv.col >= view_width && (!has_foldtext || virt_line_index >= 0)
+    if (wlv.col >= view_width && (!has_foldtext || wlv.filler_todo > 0)
         && (wlv.col <= leftcols_width
             || *ptr != NUL
             || wlv.filler_todo > 0

test/functional/ui/fold_spec.lua

@@ -2978,6 +2978,29 @@ describe('folded lines', function()
     with_ext_multigrid(false)
   end)
 
+  it('does not hang drawing diff filler above a folded line', function()
+    Screen.new(80, 24)
+    exec([[
+      call setline(1, ['fold', 'body'])
+      vnew
+      call setline(1, ['inserted', 'fold', 'body'])
+
+      windo diffthis
+      wincmd l
+      setlocal foldmethod=manual
+      1,2fold
+      normal! zM
+
+      wincmd h
+      normal! 1Gzt
+      redraw
+
+      let g:diff_filler_fold_done = 1
+    ]])
+    eq(1, api.nvim_get_var('diff_filler_fold_done'))
+    assert_alive()
+  end)
+
   it("do not interfere with corrected cursor position for 'scrolloff'", function()
     local screen = Screen.new(40, 7)
     exec([[