Updates to scrolled TabSheet resize logic and Valo right-alignment. (#11133)
- When a TabSheet is scrolled to an end and then resized bigger, more
tabs should appear to the left.
- When a TabSheet is right-aligned in Valo and scrolled to the end, last
tab shouldn't be partially hidden behind the scroller buttons.
- Shouldn't allow attempts to scroll into directions where there is
nothing left to scroll to, even if the current tab isn't fully visible.
Fixes #807
Anna Koskinen