Correct flexible box sizing calculation

/ HT @ulferts

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Extract properties for vertical scrolling to mixin

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Show only vertical scrollbars in main menu

Signed-off-by: Alex Coles <alex@alexbcoles.com>