## The basics
- [x] I [validated my changes](https://developers.google.com/blockly/guides/contribute/core#making_and_verifying_a_change)
## The details
### Resolves
Fixes#9155
### Proposed Changes
In cases when an ID is missing for an element passed to `FocusableTreeTraverser.findFocusableNodeFor()`, always return `null`.
Additionally, the new short-circuit logic exposed that `Toolbox` actually wasn't being set up correctly (that is, its root element was not being configured with a valid ID). This has been fixed.
### Reason for Changes
These are cases when a valid node should never be matched (and it's technically possible to incorrectly match if an `IFocusableNode` is set up incorrectly and is providing a focusable element with an unset ID). This avoids the extra computation time of potentially calling deep into `WorkspaceSvg` and exploring all possible nodes for an ID that should never match.
Note that there is a weird quirk with `null` IDs actually being the string `"null"`. This is a side effect of how `setAttribute` and attributes in general work with HTML elements. There's nothing really that can be done here, so it's now considered invalid to also have an ID of string `"null"` just to ensure the `null` case is properly short-circuited.
Finally, the issue with toolbox being configured incorrectly was discovered with the introducing of a new hard failure in `FocusManager.registerTree()` when a tree with an invalid root element is registered. From testing there are no other such trees that need to be updated.
A new warning was also added if `focusNode()` is used on a node with an element that has an invalid ID. This isn't a hard failure to follow the convention of other invalid `focusNode()` situations. It's much more fragile for `focusNode()` to throw than `registerTree()` since the former generally happens much earlier in a page lifecycle, and is less prone to dynamic behaviors.
### Test Coverage
New tests were added to validate the various empty ID cases for `FocusableTreeTraverser.findFocusableNodeFor()`, and to validate the new error check for `FocusManager.registerTree()`.
### Documentation
No new documentation should be needed.
### Additional Information
Nothing to add.
* feat!: Make bubbles, comments, and icons focusable
* feat!: Make ISelectable and ICopyable focusable.
* feat: Consolidate selection calls.
Now everything is based on focus with selection only being used as a
proxy.
* feat: Invert responsibility for setSelected().
Now setSelected() is only for quasi-external use.
* feat: Push up shadow check to getters.
Needed new common-level helper.
* chore: Lint fixes.
* feat!: Allow IFocusableNode to disable focus.
* chore: post-merge lint fixes
* fix: Fix tests + text bubble focusing.
This fixed then regressed a circular dependency causing the node and
advanced compilation steps to fail. This investigation is ongoing.
* fix: Clean up & fix imports.
This ensures the node and advanced compilation test steps now pass.
* fix: Lint fixes + revert commented out logic.
* chore: Remove unnecessary cast.
Addresses reviewer comment.
* fix: Some issues and a bunch of clean-ups.
This addresses a bunch of review comments, and fixes selecting workspace
comments.
* chore: Lint fix.
* fix: Remove unnecessary shadow consideration.
* chore: Revert import.
* chore: Some doc updates & added a warn statement.
This introduces new callback methods for IFocusableTree and
IFocusableNode for providing a basis of synchronizing domain state with
focus changes. It also introduces support for implementations of
IFocusableTree to better manage initial state cases, especially when a
tree is focused using tab navigation.
FocusManager has also been updated to ensure functional parity between
tab-navigating to a tree and using focusTree() on that tree. This means
that tab navigating to a tree will actually restore focus back to that
tree's previous focused node rather than the root (unless the root is
navigated to from within the tree itself). This is meant to provide
better consistency between tab and non-tab keyboard navigation.
Note that these changes originally came from #8875 and are required for
later PRs that will introduce IFocusableNode and IFocusableTree
implementations.
This adds new tests for the FocusableTreeTraverser and fixes a number of
issues with the original implementation (one of which required two new
API methods to be added to IFocusableTree). More tests have also been
added for FocusManager, and defocusing tracked nodes/trees has been
fully implemented in FocusManager.
