An interesting display problem has developed

[Watercourse]

Suddenly...
When i move any window on the screen i get a 3x3 grid that redraws itself sloooowly several times across the arc of the movement, and when it settles down the content of the window can be used again. I was playing with redshift when this began (it has has since been removed), and the only other change today was an apt upgrade or apt dist-upgrade something or other.
Is there a logfile for apt grades? There must be.
Assuming i can find the upgraded, i imagine i could roll it back but i don't know how to do either of those steps.
Sorry for the newbie helplessness, but my PBP is not really usable like this.
Anyone want to share some expertise?

Adding: Default Debian/MATE up-to-date

[SatnSpwn]

Suddenly...
When i move any window on the screen i get a 3x3 grid that redraws itself sloooowly several times across the arc of the movement, and when it settles down the content of the window can be used again. I was playing with redshift when this began (it has has since been removed), and the only other change today was an apt upgrade or apt dist-upgrade something or other.
Is there a logfile for apt grades? There must be.
Assuming i can find the upgraded, i imagine i could roll it back but i don't know how to do either of those steps.
Sorry for the newbie helplessness, but my PBP is not really usable like this.
Anyone want to share some expertise?

Adding: Default Debian/MATE up-to-date

A 3x3 grid sounds like an outline window drag. I don't have experience with MATE, but Gnome2 had a low resources mode that would enable that.

I could not find anything like this in the settings, so I installed dconf-editor. dconf-editor is like windows regedit.exe. Open it and navigate to "/org/mate/marco/general/". Look for "reduced-resources", it should be set to false. I don't see any outline window mode settings, if you see anything like that it should be set to false

[Watercourse]

Excellent! Worked a charm. Thank you very much!