As far as I can tell, this is (probably) a GTK and/or Pango bug, with really big bitmaps (e.g. 10k x 10k zoomed in several times) and enough prodding, things start to break down around the UI and sometimes we even get a crash inside GTK when drawing the wxStaticBitmap.

I think we need to roll our own bitmap control for the preview which draws directly from the file and correctly handles very large source images.