Would you be able to take a trace of the bad behavior and attach it to this bug? Instructions here:
https://www.chromium.org/developers/how-tos/submitting-a-performance-bug

-add Trace File & video recorder 

Deleted: trace_gmail.json.gz 4.5 MB

trace_gmail.json.gz 4.5 MB Download

Deleted: ScreenCaptureProject2.mp4 13.5 MB

	ScreenCaptureProject2.mp4 13.5 MB View Download

Thank you for providing more feedback. Adding requester "rschoen@chromium.org" for another review and adding "Needs-Review" label for tracking.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

I was able to repro this bug in the latest canary. More detailed repro instructions, for those who don't have RTL enabled:

1) In Gmail, click the cog menu and then Settings
2) In the very first section choose "Right-to-left editing support on" and then hit Save at the bottom
3) Go back to your inbox and open a new compose window
4) On the formatting bar, click the button to go to RTL
5) Copy and paste مرحبا and then press enter
6) On the formatting bar, click the button again to go back to LTR
7) Type hi
8) Press control-left and control-right repeatedly

Page becomes unresponsive. This seems to be an actual hang where the tab goes unresponsive, and not a performance issue. Captured a trace of the behavior where you can see the long task: https://drive.google.com/open?id=0B5eS4VhPbSBzeHN1ME1NMHZKWW8

Assigning to Tim for triage since the long task mentions input, but also cc'ing Emil in case this is a RTL layout issue.

Also tweaking the title to be more accurate

Lan, can you take a look at this? This should be a good chance to get some exposure to how keyboard input works with RTL text, which will be useful when you start thinking about keyboard gestures.

I'm bumping this up to Pri 1.

Sure, I will start soon, thanks!

Cannot reproduce anymore in Chrome 62.0.3166.0.