fix(Offcanvas): in-place panel stays interactive, focus returns on close

[xidedix]

A closed offcanvas that CSS lays out in place (responsive from its breakpoint up, .navbar-expand-* .offcanvas from the expand breakpoint up) was visible but dead: the host kept inert, role="dialog", aria-modal="true" and the hiding class from the first render, so nothing inside could be clicked or focused and screen readers announced a modal dialog. The dropdown in the dark navbar example did not open for that reason.

role and aria-modal are rendered only while the offcanvas is open, as in the vanilla edition. inert is dropped from a closed offcanvas once its computed position is read and is not fixed; until the first read, and on the server, it stays as before. The position is re-read after every render and on window resize. The hiding class is added only when an open offcanvas closes. The focus trap is enabled only while open and no longer auto-captures focus at init, which would have moved focus into an in-place panel on page load; opening focuses the offcanvas host as before.

An open offcanvas that was shown as a fixed panel is closed when a window resize lays it out in place, as in the vanilla edition. An offcanvas in .navbar-expand-* opened on a narrow window no longer stays a modal dialog with a backdrop after the window is widened. One that was never a fixed panel, for example kept position-absolute by author CSS, stays open, so a mobile soft keyboard does not close it. With portal the offcanvas sits in the container while open, stays a fixed panel and is not closed.

On close, and when an open offcanvas is destroyed, focus returns to the cOffcanvasToggle that opened it, else to the element focused when it opened. It moves only while focus is inside the offcanvas or on the body, and skips a target that is detached or cannot take focus. Returning to the toggle covers Safari, where clicking a button does not focus it. Docs: coreui/coreui-angular-docs branch fix/offcanvas-in-place.