Maps an arrow / Home / End key press to the index that should receive focus.
Pure index arithmetic — the caller owns preventDefault(), focus and any
tabindex bookkeeping, because those differ between the two widgets (the
mobile toolbar swaps tabindex on move, the desktop tab strip only focuses).
Parameters
key: string
The KeyboardEvent.key value.
currentIndex: number
Index of the currently focused item, already validated as in-range.
count: number
Number of focusable items; must be ≥ 1.
Returns number|null
The target index, or null when the key is not a navigation key.
Maps an arrow / Home / End key press to the index that should receive focus.
Pure index arithmetic — the caller owns
preventDefault(), focus and anytabindexbookkeeping, because those differ between the two widgets (the mobile toolbar swapstabindexon move, the desktop tab strip only focuses).