Content Analysis needs to be able to show a tab-modal dialog in the
sidebar, in case users paste text into a GenAI chatbot, etc. The dialogs
need to be tab-modal because the "DLP busy" dialog needs to be closable
by the JS code when the operation is done.
This change detects whether a dialog is trying to be displayed from a
browser inside the sidebar and shows it correctly.
Note that this does not change how extensions behave; if they try to
display a tab-modal dialog it will end up being window-modal (see
discussion in bug 1680066 and bug 1513656). I had a patch to fix this
but ran into some window leaks during tests that I was unable to address.
Differential Revision: https://phabricator.services.mozilla.com/D242957
036a0bca8e