fix(terminal): scrollback may still be wrong on height increase (#37835)

Problem:  Terminal scrollback may be wrong when increasing height after
          outputting lines with full scrollback.
Solution: Ensure enough number of scrollback lines have been deleted.

src/nvim/terminal.c

@@ -2397,19 +2397,18 @@ static void refresh_scrollback(Terminal *term, buf_T *buf)
   mark_adjust_buf(buf, 1, deleted, MAXLNUM, -deleted, true, kMarkAdjustTerm, kExtmarkUndo);
   term->old_sb_deleted = term->sb_deleted;
 
+  int old_height = term->old_height;
   int width, height;
   vterm_get_size(term->vt, &height, &width);
 
-  int max_line_count = (int)term->sb_current - term->sb_pending + height;
-  // Remove extra lines at the top if scrollback lines have been deleted.
-  while (deleted > 0 && buf->b_ml.ml_line_count > max_line_count) {
+  // Remove deleted scrollback lines at the top, but don't unnecessarily remove
+  // lines that will be overwritten by refresh_screen().
+  while (deleted > 0 && buf->b_ml.ml_line_count > old_height) {
     ml_delete_buf(buf, 1, false);
     deleted_lines_buf(buf, 1, 1);
     deleted--;
   }
-  max_line_count += term->sb_pending;
 
-  int old_height = MIN(term->old_height, buf->b_ml.ml_line_count);
   while (term->sb_pending > 0) {
     // This means that either the window height has decreased or the screen
     // became full and libvterm had to push all rows up. Convert the first
@@ -2422,6 +2421,7 @@ static void refresh_scrollback(Terminal *term, buf_T *buf)
     term->sb_pending--;
   }
 
+  int max_line_count = (int)term->sb_current + height;
   // Remove extra lines at the bottom.
   while (buf->b_ml.ml_line_count > max_line_count) {
     ml_delete_buf(buf, buf->b_ml.ml_line_count, false);

test/functional/terminal/scrollback_spec.lua

@@ -1263,6 +1263,25 @@ describe('scrollback is correct', function()
           check_buffer_lines(0, 99)
         end)
       end
+
+      describe('with full scrollback,', function()
+        before_each(function()
+          api.nvim_set_option_value('scrollback', 6, { buf = buf })
+          check_buffer_lines(82, 94)
+        end)
+
+        it('output first', function()
+          command(send_cmd .. ' | resize +2')
+          screen:expect(screen_final)
+          check_buffer_lines(87, 99)
+        end)
+
+        it('resize first', function()
+          command('resize +2 | ' .. send_cmd)
+          screen:expect(screen_final)
+          check_buffer_lines(85, 99)
+        end)
+      end)
     end)
 
     describe('decreases in the same refresh cycle as outputting lines', function()