Clean up some Conpherence beahviors with a mouse

Summary:
Fixes T7825. If JX.Scrollbar activates, we sometimes target the wrong node.

(We don't have this issue in the column because it rebuilds a new JX.Scrollbar every time.)

Test Plan:

• Sent messages, no spooky text.
• Loaded page, got scroll to bottom.
• Unplugged all USB devices, restarted browser, repeated.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7825

Differential Revision: https://secure.phabricator.com/D12413