a11y: Settings UI

[cameron1024]

Adds:

• aria attributes to most UI elements in settings UI
• a new a11y API to GPUI
• a fix for a GPUI keyboard focus bug

Accesible settings UI

Settings UI should now be fully accessible to users of assistive technology.

However, there are some caveats:

• accessibility features require zed to be launched with the ZED_EXPERIMENTAL_A11Y=1 env var to be set
• I have not exhaustively checked every control
• There are some quite surprising keyboard focus behaviours which predate this code
• The main Zed UI is still largely inaccessible, though a handful of shared components will now report themselves, but the experience is suboptimal.

For anyone wishing to try out the settings UI:

• make sure ZED_EXPERIMENTAL_A11Y=1 is set
• open zed
• press ctrl-, (or cmd+, on a mac) to open settings UI in a separate window

aria_active_descendant

This is equivalent to the aria-activedescendant API on the web.

It allows a container to maintain focus, while indicating that one of its children should be active. Zed uses this for menus and combo boxes, for example.

GPUI will report a div with .aria_active_descendant() as long as:

• an ancestor is focused
• only one descendant of the focused ancestor has .aria_active_descendant()

Two descendants with this property where the focused node is a common ancestor is an error.

GPUI keyboard fix

GPUI will map a space or enter keypress to a div's on_click handler, if it exists. However, the keyboard-driven path was missing some checks that the mouseclick path has.

With this PR, for an enter/space keypress to be considered a click, between the "key down" and "key up" events:

• there must be no other key events
• focus must not move to a different node

This fixes an existing bug with combo boxes in zed where:

• enter maps to menu::Confirm *on key down
• the menu item is selected
• focus moves to the combo box
• enter then triggers the combo box's on_click on key up, re-opening the menu

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Release Notes:

• N/A or Added/Fixed/Improved ...