Offcanvas: activate focustrap when backdrop is enabled

[GeoSot]

closes #36596

Preview https://deploy-preview-36717--twbs-bootstrap.netlify.app/

/cc @patrickhlauke

[julien-deramond]

Added 658045e to tackle the 3rd point from the issue:

Also noticed that focus is not moved to either of the off-canvases (the docs nav, and the general nav), but remains on the toggle buttons. Not looked at the markup in depth, but suspect a tabindex="-1" is missing on them (compare to the working off-canvas examples https://getbootstrap.com/docs/5.2/components/offcanvas/).

To test it in the docs, simply focus the main header (keyboard), open both menus with 'Enter' and then press to 'Escape' to close them. 'Escape' didn't do anything before since offcanvases didn't get the focus. Now it's working.

[julien-deramond]

If I have well understood the 2nd point in the issue, it is safe to remove aria-expanded="false" from the .navbar-togglers in the docs since offcanvases won't change the status and make an aria-expanded="true".
Done via d8f192d.

[julien-deramond]

JS part LGTM! I've added 2 commits to handle 2 other points mentioned in the issue. This PR will need also a review from @patrickhlauke.

[patrickhlauke]

Looking good, and correctly addresses the issue #36596

At a higher level though, I'm wondering if we should just always enable focus trapping? Or, if we do want to allow non-modal offcanvases, we should document this clearly in the docs (currently it seems the docs are silent about focus behaviour at all) and also make sure that aria-modal="true" is only added when it's intended to be a modal offcanvas

[GeoSot]

At a higher level though, I'm wondering if we should just always enable focus trapping? Or, if we do want to allow non-modal offcanvases, we should document this clearly in the docs (currently it seems the docs are silent about focus behaviour at all) and also make sure that aria-modal="true" is only added when it's intended to be a modal offcanvas

Either way, an offcanvas that doesn't scroll or has a backdrop, behaves like modal, so I think is right for know.
BUT yes, we may, can discuss it for next releases