Upstream https://bugs.chromium.org/p/gerrit/issues/detail?id=8812

To reproduce:

1. Use Gerrit new UI (polygerrit)
2. Open a change and click "Reply".
3. Type foo bar.
4. Select the a in bar
5. Type z. The text now reads "foo bzr"
6. Type ⌃ + Z, which in normal text areas is "undo"
7. Observe that the change was not undone: the text is still foo bzr

Expected behavior:
In step 6, the text would again read "foo bar".

Additional notes:

This is with the new Gerrit UI (polygerrit), the old UI (GWT based) works as expected.

You could then continue by pressing ⌃ + ⇧ + Z (or ⌃ + Y, at least in my version of Firefox) which would redo the change to produce foo bzr.

You should also be able to type a long paragraph, making many edits, and use ⌃ + Z and ⌃ + ⇧ + Z to repeatedly undo and redo, as is the case in a standard textarea.

For whatever reason this bug does not occur when making an inline comment, only when using the reply popup.