Fix inline reticles and dialogs for new scrollbars

Summary:
Fixes T7054. Fixes T7049.

• Stop scrolling Differential reticles when the page scrolls.
• Make dialogs aware of multi-panel UI.

Test Plan:

• Dialogs pop up in the right place.
• Inline + scroll now longer leaves the inline in a fixed position.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7049, T7054

Differential Revision: https://secure.phabricator.com/D11523