Chrome Version: 56.0.2924.21
OS: MacIntel / Ubuntu

Reported internally:

I was able to get Gmail to freeze consistently by copying test from Google Docs, pasting it into Gmail, and then trying to make that list bulleted.

The text from docs did contain strikethrough text if that helps.

Additional Repro steps that hit the same issue:

1. Navigate to https://jsfiddle.net/ganugLrp/
2. Copying the content-editable text
3. Paste it into a gmail compose window
4. Select the tests
5. Pressing the editor button to make a bullet list