Git bisect shows that this started at https://chromium-review.googlesource.com/c/chromium/src/+/791031, which is related to mouse events:

5a995c5c20a24a83b1aa3cddb28076c8007ca01d is the first bad commit
commit 5a995c5c20a24a83b1aa3cddb28076c8007ca01d
Author: Dave Tapuska <dtapuska@chromium.org>
Date:   Mon Nov 27 20:23:22 2017 +0000


dtapuska, I'm assigning this to you as the owner of that change.

Here's how you reproduce:
1. Enable select-to-speak in chrome://settings/manageAccessibility
2. Hold the search key, and click and drag and release the mouse to draw a rect.

Expected behavior: Focus ring draws smoothly
Actual behavior: Focus ring only drawn at mousedown/mouseup events, but mousemove events are not passed to the Select-to-Speak event handler.

Note that select-to-speak runs in a chrome extension in the background. If I simply add mousemove listeners to a normal document it works fine, but adding more mousemove listeners to the Select-to-Speak extension never gets them fired.

Marking as P1 because this makes select-to-speak seem frozen or unusable and there's a branch point coming up.

Ugh, I see the flaw. The background extension doesn't generate a frame sink. Should be an easy fix tomorrow.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fb79dc9defdfa3ea23441bd7a14404d862952137

commit fb79dc9defdfa3ea23441bd7a14404d862952137
Author: Dave Tapuska <dtapuska@chromium.org>
Date: Wed Nov 29 13:35:23 2017

Fix rAF aligned mouse input in background extensions.

Change https://chromium-review.googlesource.com/791031
missed a variable that was switching off the initialization of
mouse rAF aligned events. Keep the variable around since
it seems to be used for extensions.

BUG= 703344 , 789336 , 789096 
TBR=mustaq@chromium.org

Change-Id: Ia9e23bc59fa9903f1413ca43e153843280d7805d
Reviewed-on: https://chromium-review.googlesource.com/795451
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520093}
[modify] https://crrev.com/fb79dc9defdfa3ea23441bd7a14404d862952137/content/renderer/input/main_thread_event_queue.cc
[modify] https://crrev.com/fb79dc9defdfa3ea23441bd7a14404d862952137/content/renderer/input/main_thread_event_queue.h

Thanks for the quick fix! I verified that it's working again at HEAD.