Well, thanks @mouser, but don't be in too much of a hurry to apologise just yet.
I am guessing that no-one else has had this error - is that right?
If it's just me, then that "invalid pointer operation" might be a clue.
The touchpad/mouse has been behaving badly - very sluggish on scrolling - but only in Firefox, it seems. This is something new, starting a couple of days ago.
Otherwise the pattern seemed to be:
1. Selecting the arrow and text box tools, and then the object selector last seemed to trigger the error.
2. The error is always a light blue screen with only one or two objects from your editing (but no text in the boxes). Trying to Undo the last change seems to trigger the error box "Invalid pointer operation" error message, and it seems that at that point the screenshot gets overwritten. All changes lost, anyway.
I can't repeat it every time, so it's not repeatable.
I worked around it by taking a copy of the amended screenshot image after every small step, and then using the copy to replace the working copy in the Screenshot directory, every time the working copy was zapped/overwritten by that blue background image file.