Allow keyboard navigation from highlight in host frame to associated annotation in sidebar about client HOT 3 CLOSED

We already have a "Show" button that appears in the toolbar when already-highlighted text is selected, so one possibility is that when this button is triggered, we not only filter annotations in the sidebar (as we do now), but also transfer keyboard focus to the sidebar and to the annotation card for the first annotation in the selected set. Assuming the user has a way to select text with the keyboard, this provides a keyboard-only way to navigate from the highlight to the annotation in the sidebar.

In addition to focus transfer when clicking "Show", we'd also need to announce the "Show" shortcut. Perhaps the solution we land on for #4742 can be extended to include this.

A closely related thing we need to think about is how the user gets back to where they were in the document after they jump to the sidebar. My current thought is that we'll have some shortcut in the sidebar that returns focus back to the host/guest frame where focus was previously, and focuses the highlight that the user jumped from.

from client.

One issue I ran into while implementing this is that the client is currently a bit trigger happy when it comes to selecting annotations. The selection in the sidebar is changed not only when clicking on highlighted text, but also whenever the cursor is released inside a highlight. The "Show" action in the toolbar currently just triggers an update of the selection. If we're going to make it also move focus to the sidebar, we need to make sure that doesn't happen when eg. making a text selection that happens to end inside a highlight.

from client.

We already have a "Show" button that appears in the toolbar when already-highlighted text is selected, so one possibility is that when this button is triggered, we not only filter annotations in the sidebar (as we do now), but also transfer keyboard focus to the sidebar and to the annotation card for the first annotation in the selected set

I have this working locally. It works as intended in Chrome and Firefox. In Safari there is a complication that the sidebar cannot request focus until the user has interacted with it at least once since the top-level frame loaded. Prior to that window.focus() calls inside the sidebar are ignored, and if the host frame attempts to call focus() on the iframe element, the frame element is focused, but not the window it contains.

from client.