You were absolutely right - it IS the magnifier window setting!!! I'm thinking... hmm... YOUR post is the one that actually solves the problem! Thank you!
Since I disabled that setting, it's been working fine! What an annoying mess, this whole thing has been... so glad it's over. At some point, it just felt easier to sell the phone than to fix the issue.
I used the magnifier window pretty often, too, so... I'm not sure whether we've won or lost, but at least it's figured-out!