[stumped]

All I can offer for clues is this.
Normally I only. Modify books just added to my category I, in progress.
A short list
. Works fine
I.e.i choose something new to read. Mark it as I, in progress, then use modify as a final prep of a reading version prior to the actual read. I scrub any unwanted front and back of book stuff , to go straight to the story, and let the plug in fix the toc to match

Bug appears if I scroll through a longer list e.g. my not read, read =n, list then pick out a book that I probably never tidied, not on screen 1 of that list. It is only after modifying something way down that list that I lose focus.afterwards
I will experiment more tomorrow, and post any replicable sequence of events

I suspect the plug in is partly to blame. It exits and tells calibre to go back to showing the list from which it was activated, but does that in a manner that causes calibre to show screen 1 of a multi screen filter list. So the book it just modified is off screen. I have about 400 books on that unread iist, so many many screens. I need to test more carefully and confirm that , though