fix(folds): fix fold remains when :delete makes buffer empty (#19673)

src/nvim/change.c

@@ -421,12 +421,12 @@ void deleted_lines(linenr_T lnum, linenr_T count)
 /// be triggered to display the cursor.
 void deleted_lines_mark(linenr_T lnum, long count)
 {
-  // if we deleted the entire buffer, we need to implicitly add a new empty line
   bool made_empty = (count > 0) && curbuf->b_ml.ml_flags & ML_EMPTY;
 
-  mark_adjust(lnum, (linenr_T)(lnum + count - 1), (long)MAXLNUM,
-              -(linenr_T)count + (made_empty?1:0),
-              kExtmarkUndo);
+  mark_adjust(lnum, (linenr_T)(lnum + count - 1), MAXLNUM, -(linenr_T)count, kExtmarkNOOP);
+  // if we deleted the entire buffer, we need to implicitly add a new empty line
+  extmark_adjust(curbuf, lnum, (linenr_T)(lnum + count - 1), MAXLNUM,
+                 -(linenr_T)count + (made_empty ? 1 : 0), kExtmarkUndo);
   changed_lines(lnum, 0, lnum + (linenr_T)count, (linenr_T)(-count), true);
 }
 

test/functional/ui/fold_spec.lua

@@ -1904,4 +1904,11 @@ describe("folded lines", function()
   describe('without ext_multigrid', function()
     with_ext_multigrid(false)
   end)
+
+  it('no folds remains if :delete makes buffer empty #19671', function()
+    funcs.setline(1, {'foo', 'bar', 'baz'})
+    command('2,3fold')
+    command('%delete')
+    eq(0, funcs.foldlevel(1))
+  end)
 end)