R1:24ba66f106f2

Persist "Show Changeset" and improve path text selection

Summary:
Ref T13513. Currently:

- If you click the "Show Changeset" button, your state change doesn't actually get saved on the server.
- It's hard to select a changeset path name for copy/paste because the "highlight the header" code tends to eat the event.

Instead: persist the former event; make the actual path text not be part of the highlight hitbox.

Test Plan:
- Clicked "Show Changeset", reloaded, saw changeset visibility persisted.