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.
  - Selected changeset path text without issues.
  - Clicked non-text header area to select/deselect changesets.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21236