Smoothier transition for frame between different-width thumbnails

Right now when you have thumbnails with different width and you switch between them, the frame takes the width of a new thumbnail momentarily. If you'd add a transition to width there, the switch would be a bit more visually smoothier.

However, if there are reasons not to have this transition, it's ok to close this PR :) Also, I'm not sure I placed the fix into the right place — the name of the placeholder confuses me a bit, however, it's fitting perfect for the goal, and there shouldn't be any side effects.