It appears that this was introduced for high-DPI screens in this CL:
https://codereview.chromium.org/402283003

and low-DPI in this CL:
https://codereview.chromium.org/2869813002

Both CLs were fixing correctness issues, but this fix is overly broad.
There should be a way to avoid the composited layer when not actually needed
for other reasons.

I think my TODO in CompositingReasonsFinder is talking about this situation:
      // TODO(schenney): We only need to promote when the clipParent is not a
      // descendant of the ancestor scroller, which we do not check for here.
      // Hence we might be promoting needlessly.

Not sure if that is the correct analysis, however.

Ping - chrishtr: what's the plan? Any recent updates?

This is a case of compositing too much, right, not correctness of the painted result. Fixing it will probably wait until the we are through with Slimming Paint V2 and only need a single fix.

There has been no progress in the last month or two. I think we will likely
have to wait for SPv2, but plan to try to untangle a bit sometime in the next
3 months.

 Issue 842258  has been merged into this issue.