Word-block count and table of contents popover accessibility

[afercia]

Follow up to #2684. Thought there was already an issue open for this but couldn't find it. Feel free to close this if I've missed it.

There are still some issues to address:

• placement of the "I" button as discussed on Add word and block count to table of contents #2684 and Open/close sidebar interaction is un-intuitive #3166
• the toggle button "I" is empty and misses any accessible name. It used to have a visible "Info" text that was removed in 7e3cc89 without providing any alternate name to the button. As a consequence, there's nothing assistive technologies can announce other than "button"
• the toggle button should have an aria-expanded attribute
• it's an "icon-only" button, I wonder if it should use a tooltip as similar buttons do
• focus management: this is a general issue for the Dropdown (formerly "PopOver") component

[mtias]

@afercia do we have pending items here?

[afercia]

@mtias couple issues to consider:

1
When there are headings, opening the "Content Structure" move the initial focus to the first heading in the ToC. Not so ideal because all the previous content (the counts) get skipped.

2
Tabbing should be constrained within the Dropdown (formerly "PopOver") but I guess this should be addressed in that component. Is there already a specific issue for that?

I've noticed also a couple layout/usability potential problems, going to open a new issue.

[afercia]

Many things have changed here, I think this can be closed now.

• about constraining tabbing within the Popover, there's a specific issue see Constrain tabbing within popovers and similar components #5242
• about initial focus when there are no headings, I'm going to push a PR

[mtias]

about initial focus when there are no headings, I'm going to push a PR

Cool. I was thinking of a general solution as well where we can move focus to a popover but don't have to focus any specific item within it.

[afercia]

Hm the pattern recommended by the ARIA authoring practices is to always move focus to the first focusable element inside a popover or menu. We're discussing this in a separate issue, see #5559

[mtias]

We have gotten some feedback from users who don't expect a first item to be selected and confuse the focus ring with a heading style.

I think the prevalence of menus like these, that focus on the thing that opened the menu, contribute to that feeling:

[afercia]

Please refer to the ARIA resources posted on #5559. Moving focus to the first item is a requirement. We shouldn't try to solve what is a styling issue at the expenses of a required functionality.

[mtias]

I don't think it's a styling issue.