While loading items for the sidebar in single image view mode, show progress instead of "(Empty)" which is misleading
If, from Nautilus, in a cluttered folder with many files (like your ~/Downloads folder; mine has over 500 files in it), you open an image file for GThumb to display, GThumb shows the "(Empty)" label in the sidebar:
In many cases I've thought there was a bug and GThumb was unable to show items in the folder. Well, this is a lie. A dirty, dirty lie! Behold, a little while later:
So, presuming you don't want to embed a Gtk Progressbar widget in there, I think a very simple and effective solution would be to set the label to "Loading (CURRENT_ITER) of (TOTAL)…", ex.: "Loading 45 of 544…". No need for super accurate percentages, and updating that GtkLabel widget should be super cheap in terms of performance.
Keep the "(Empty)" label only for when the folder is truly empty.