This looks relatively recent:

Dev 68.0.3418.2 does not have this problem

But 

Canary Version 68.0.3422.0 does.

ccameron@ and I both landed patches that could cause this:

I landed https://chromium-review.googlesource.com/c/chromium/src/+/1039768

Surface synchronization: Simplify resize throttling (first appeared in 3420)

ccameron@ landed:

https://chromium-review.googlesource.com/c/chromium/src/+/1046045

viz/mac: Fix surface invariants violation by recycled compositors (first appeared in 3422)

I just tested 68.0.3420.0 and verified that it works fine. I'm now suspecting ccameron@'s CL, so I'm assigning to ccameron@

I'm going to try building locally and reverting ccameron's CL.

Doh, looks like I caused it here:

https://chromium-review.googlesource.com/c/chromium/src/+/1011221

Surface synchronization: Report updated child allocated IDs to parent.

Not yet sure how given I didn't touch any Mac specific code here. Investigating and assigning the bug to myself.

Ahh I think RenderFrameMetadataObserverClient binding needs to be attached to the WindowResizeHelper's task_runner.

I have a fix: https://chromium-review.googlesource.com/c/chromium/src/+/1046003 Passed along to ccameron@

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/20aeb5912ae7b8fdf58e073d948ed347cebcba42

commit 20aeb5912ae7b8fdf58e073d948ed347cebcba42
Author: Fady Samuel <fsamuel@chromium.org>
Date: Mon May 07 14:19:43 2018

Surface Synchronization: Fix Mac regression

After Visual Properties ACK (Resize ACK) was moved to the
RenderFrameMetadataObserverClient interface, Mac web content no longer
stuck to the edge of the window. This happened because WindowResizeHelper
didn't see RenderFrameMetadata tasks and so did not schedule them until
AFTER the resize deadline hit.

This CL moves RenderFrameMetadataObserverClient tasks to the
WindowResizeHelper's task_runner thereby fixing resize on Mac.

Bug:  840197 ,  672962 
Change-Id: If4f8a766a2322f1208b6ffccca89c5181f96bf8f
Reviewed-on: https://chromium-review.googlesource.com/1046003
Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: Fady Samuel <fsamuel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556432}
[modify] https://crrev.com/20aeb5912ae7b8fdf58e073d948ed347cebcba42/content/browser/renderer_host/render_frame_metadata_provider_impl.cc
[modify] https://crrev.com/20aeb5912ae7b8fdf58e073d948ed347cebcba42/content/browser/renderer_host/render_frame_metadata_provider_impl.h
[modify] https://crrev.com/20aeb5912ae7b8fdf58e073d948ed347cebcba42/content/browser/renderer_host/render_widget_host_impl.cc