This is working fine for me on Chrome 63.0.3239.84 (Linux)
Do you have any extensions installed that could be interfering with this?
What hardware is this being reproduced on?

Are you able to provide a screen recording showing it?

I found the culprit! The issue appears with flag enable-site-per-process (Strict Site Isolation). I run Canary with the same flag, and there is no issue there, so I guess it was resolved after M63.

Thanks, your request to check extensions made me check my flags as well.

Adding Internals>Sandbox>SiteIsolation per #2, and adding a few more people familiar with input events for OOPIFs.  Is it expected that we're breaking this for M63, and do we know what fixed it?

veranika@: can you please still check if you had either the Hangouts app or extension (or both) installed when you hit this?

It was *likely* resolved here:

https://chromium-review.googlesource.com/c/chromium/src/+/818118

I'm going to mark this is wontfix (obsolete), but if it reproduces on M64 canary, or tip-of-tree, please re-open and assign to me.