Scrollbar redraw issue on resize with snapshot-154 #848

After upgrading from snapshot-153 (Vim-8.1.577) to snapshot-154 (Vim-8.1.873), an issue with scrollbar rendering has appeared when resizing a MacVim window to make it larger.

This seems to be related to #802, but the trigger is different.

• Mac OS X 10.11.6 (15G22010)
• MacBook Pro (Retina, 13-inch, Early 2015)
• External DisplayPort monitor (27-inch (2560 x 1440))

If resizing by mouse, dragging the right edge of the MacVim window to the right (bigger) leaves a ghosted scrollbar near the right edge; this happens whether the window contains a new, empty buffer or a buffer with text larger than the window, and it happens regardless of whether I drag fast or slow.

If I use SizeUp or Spectacle to resize to the left or right half of the display via a hotkey, the behavior is more pronounced -- the ghost scrollbar appears where it was before the resize, and only as tall as it was before the resize.

In both cases, redrawing the window via Control+L makes the ghosted scrollbar disappear.