[Quoth]

Some ebooks (few) display cover when opened and then the thumbnail works. If the cover isn't displayed on opening, then the thumbnail is later blank instead of the Generic icon.

You don't seem to be able to add books to favourite from Search or Library but only when the book is open.

The "Back" button does return you to position on search list.

There are page numbers & total pages at the header. It's not per-screen so unaffected by font size or margins. So both system TOC and the logical page numbers (Goto Page) work to navigate. The "Back" button only returns to last page if a link was followed, otherwise it goes back to screen where you opened the book.

Images with no scaling are indeed 1:1, so QR codes work on my phone baode scanner even if set to native pixel size in the CSS.

This will be simpler and faster to test ebooks for sanity/compatibility than my ancient 4.3″ screen Sony-Ericsson Android 4 phone or the ancient 5″ Sony PRS-350 (Why is battery not dead yet?) as it's very fast to write to from Calibre.