Native leftarrow and rightarrow
Use native cursoring, in order to handle grapheme clusters and BIDI correctly.
Fix up the native behaviour if it steps over a focusable node.
Also fix up the native behaviour if it quirkily steps *into* a focusable node
(in spite of the node having ce=false).
- Use EventSequencer afterOne instead of setTimeout in up/down handling
- Abort selection fixup if we haven't landed in a view node
- Selection extension logic for a focused node
This change exposes certain browser quirks relating to cursoring.