UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.71 Safari/537.36

Steps to reproduce the problem:
Repro Steps:
1. Find an email in your Pinto mailbox created from comments replies to you
https://screenshot.googleplex.com/ACnmhkOjWwG
2. Click on Reply button - you'll get a tear-off Compose window
3. Remove all the text in the opened window using ctrl+A and hitting delete or backspace
4. Type some text with a space, e.g.
my text

What is the expected behavior?
 you see 'my text' typed

What went wrong?
you see 'mytext' typed (white space between "my" and "text" is removed.

Did this work before? Yes Mac/Chrome53

Chrome version: 54.0.2840.71  Channel: n/a
OS Version: 10.0
Flash Version: Shockwave Flash 23.0 r0

I can also repro the issue on Mac/Chrome54. 
Issue not reproducible on Mac/Chrome53.