Version: found on 53.0.2764.0 Linux

What steps will reproduce the problem?
(1) Open DevTools, then DevTools in DevTools, (undock the first DT, then Cmd+shift+I)
(2) In the first DevTools, go to Console and enter enough lines to see a scrollbar
(3) Turn on 'Paint Flashing' in the Rendering drawer in the second DevTools
(5) Start typing in the first DevTools console's prompt

Observe that on each keystroke all the previous messages are flashing for repaint.  For some reason, this doesn't seem to occur on 53.0.2768.0 Mac.

Bisect led to this commit related to scroll updates
https://chromium.googlesource.com/chromium/src/+/aff9f64394945f24400021fdec18dbd70eee3e14


I've made a standalone JSFiddle with what I believe to be the root issue here:
What steps will reproduce the problem?
(1) Go to https://jsfiddle.net/2jLp67d2/1/
(2) Open DevTools, open the Rendering tab from the drawer
(3) Enable Paint Flashing

What is the expected output?
On stable, the only repainted area is the 'dynamic' div.

What do you see instead?
On dev, according to the Paint Flashing, the entire gray box seems to be repainted, even though there is just one dynamic div.