it does have CTI tests but doesn't show display
Ah OK. Yours has an LCD display mine is an Oled / Amoled. I can't see why that makes a difference but it just displays three single colours in order so maybe it's an Oled colour test thing.
What you describe does ring a bell, but I can't remember anything about it. Was it a rectangular box with rounded corners. It may come back to me.
As B. Diddy suggested, try leaving it in Safe mode for a while when you don't need to use downloaded apps. It still functions as a smartphone.
You can screen record as a video, well mine can, and that would be helpful if you can catch it then.
I think that's what, again, B. Diddy was referring to. The white part may or may not appear in a screen recording or a screenshot though.
If I remember anything I'll post here.
*also you may have seen on the CIT screen there is an overflow menu of other "tools" with the 3 dots upper right, but Display is unlikely to be there.
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