feat: Remove main workspace tree/treeitem & use top-level regions (experimental)

[BenHenning]

The basics

• I validated my changes

The details

Resolves

Fixes #9395
Fixes RaspberryPiFoundation/blockly-keyboard-experimentation#428
Fixes part of #9312

Proposed Changes

Removes all tree and treeitem roles from non-flyout workspaces, and uses region for top-level workspace, toolbox, and flyout roles.

Demonstration of changes:

Screen.recording.2025-10-08.10.37.10.AM.webm

(This shows the reintroduction of block role description, the new regions, the removal of tree/tree items for blocks, and an attempt at fixing mutators though with apparent issues).

Reason for Changes

Recent feedback from micro:bit panelists and accessibility experts suggests that the tree-based approach may be adding too much contextual information to the point of not actually being helpful (and potentially even being harmful from a UX perspective). Additionally, VoiceOver has mixed support for trees and tree items and it may be very difficult or impossible to work around these and continue with those roles.

The new plan is to test with less explicit context and add some more context through block labeling (particularly for C-shaped blocks), which is ongoing work in #9299. Longer term it may be necessary to add a more 'verbose' readout mode, or add some sort of 'where am I?' shortcut to re-add the contextual positioning information (so the code removed in this PR may be useful for future referencing).

Some specific notes on regions:

• Regions provide explicit readout context (at least in ChromeVox) usually when entered and exited. This actually makes it possible to leverage both regions and trees for context (in the case of toolboxes and flyouts) which is actually quite clean.
• Mutators are still very fuzzy. Mutators are being made a region since they are technically distinct workspaces from the main workspace, but the readouts for them on ChromeVox aren't working as expected which is why Improve aria-label for mutator workspaces #9312 isn't being fully closed here. More work is needed to get mutators to have a good readout experience.

Note also that this reintroduces the 'block' role description for the main workspace and is leveraging figure (https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Reference/Roles/figure_role) as the base role for blocks. This seems like the best fit as blocks are not inherently buttons, and overwriting the role description is supposed to provide the correct context to readers. This may well need to change if more incompatibilities are discovered (such as with VoiceOver).

Test Coverage

None since this is an experimental change.

Documentation

None since this is an experimental change.

Additional Information

None

[maribethb]

I think it's kind of weird for the flyout workspaces to still be treeitems. The categories in a toolbox make sense as trees, but blocks in a flyout should probably be treated like blocks in the workspace, in my opinion.

[BenHenning]

Yeah that seems right. It seems maybe useful to treat it like a list of some kind, but we may be stretching the definition of list item here. I can just go back to the idea of no list entirely and see what testing brings up.

[gonfunko]

Talked with @rachel-fenichel and decided to merge now to unblock further work.