Hi Gregor, I'm very happy that you decided to implement a non-popup sub-variation traversal. I also like your keyboard navigation, but my favorite navigation style is the "one-finger-wheel-navigation" in scid.
1. d4 d5 ( 1... Nf6) ( 1... g6) 1) Wheel-down navigates to d5 and shows arrows for d5 (main) NF6 g6 2a) Wheel down follows main variation (d5) 2b) Click on target square navigates inside a variation. E.g. g6 3) Wheel-up navigates to begin of variation g6. 4) Wheel-up again navigates back to d5 position including arrows. 5) Click on NF6 opens first variation ... What do you think? Cheers Stefan