## The basics
- [x] I [validated my changes](https://developers.google.com/blockly/guides/contribute/core#making_and_verifying_a_change)
## The details
### Resolves
Fixes https://github.com/google/blockly-keyboard-experimentation/issues/515
### Proposed Changes
This adds `canBeFocused()` checks to all the places that could currently cause problems if a node were to return `false`.
### Reason for Changes
This can't introduce a problem in current Core and, in fact, most of these classes can never return `false` even through subclasses. However, this adds better robustness and fixes the underlying issue by ensuring that `getFocusableElement()` isn't called for a node that has indicated it cannot be focused.
### Test Coverage
I've manually tested this through the keyboard navigation plugin. However, there are clearly additional tests that would be nice to add both for the traverser and for `WorkspaceSvg`, both likely as part of resolving #8915.
### Documentation
No new documentation changes should be needed here.
### Additional Information
This is fixing theoretical issues in Core, but a real issue tracked by the keyboard navigation plugin repository.
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.
This is the bulk of the work for introducing the central logical unit
for managing and sychronizing focus as a first-class Blockly concept
with that of DOM focus.
There's a lot to do yet, including:
- Ensuring clicks within Blockly's scope correctly sync back to focus
changes.
- Adding support for, and testing, cases when focus is lost from all
registered trees.
- Testing nested tree propagation.
- Testing the traverser utility class.
- Adding implementations for IFocusableTree and IFocusableNode
throughout Blockly.
