Chrome Version: 64.0.3274.0 (Developer Build) unknown (32-bit)
OS: Android N

What steps will reproduce the problem?
(1) Set up an Android device with Gboard and also a physical keyboard. Enable showing the on-screen keyboard while the physical keyboard is connected.
(2) Go to editpad.org and type a few words (on either keyboard).
(3) Repeatedly press Ctrl-Z on the physical keyboard to undo the typing.

What is the expected result?

Undo should work as it does on other platforms, where undo removes several words (e.g. a sentence) at a time.

What happens instead?

Undo undoes one character at a time, and causes the word with a character being removed from it to become selected.

This happens because InputMethodController::SetComposition() works by selecting the composition range (which closes out the previous TypingCommand, putting it on the undo stack; this is why undo happens one character at a time), replacing the selection, and then converting the selection back to a composition range (without updating the selection in the open TypingCommand; this is why undo selects the words).

I tested typing in Japanese on macOS and similar behavior resulted.

This bug would affect Android particularly badly (since pretty much all on-screen keyboards use composition ranges for composing text, even for alphabetic languages), except the only way I know of triggering undo on Android is pressing Ctrl-Z on a physical keyboard. If we ever want to expose undo in the UI, fixing this bug will become high-pri.