Combobox: keyboard selection skips disabled options inconsistently

Steps to reproduce

1. Open the Combobox demo with a list containing disabled options
2. Focus the input and press ArrowDown repeatedly
3. Reach a disabled option surrounded by two enabled ones

Expected

Focus skips the disabled option and lands on the next enabled one, in both directions.

Actual

Going down skips correctly, going up stops on the disabled option and the aria-activedescendant points to a non-interactive element.

Likely an off-by-one in findNextEnabledIndex when iterating backwards.