Steps to Reproduce:
Enable "Load last eComics Read" "Comic in its own Window" and "Full Screen" options.
Load unregistered ComicRack with a previously opened comic

What Happens:
Comic loads in full screen, covering all other windows. Donation window blocks all clicks within ComicRack. Result is fullscreen comic window that does not accept any clicks, and covers all other windows (including main window and donation window).

What Should Happen:
When spawning the donation window, ComicRack should delay loading the last comic until the donation window is no longer open.

Yep, but try to press [Tab] key three times and then hit the [Space]. This should dismiss Donation Window.

Other way would be [LeftAlt+Space], [Arrow Up], [Enter].

Hitting ESC should also work.
I'll check it out.