A recent bug fix involved moving part of an `editor-canvas%' resize
out of atomic mode by queueing a callback (because the resize involves
quesrying the editor for its size, etc.), but then the callback
happens after a canvas is shown, which can cause it to appear with
bogus initial scrollbars. Queue the callback instead as a "refresh"
level callback, which gets a chance to run before a frame is made
visible.
