EVALUATION

The overlapping isue and the left/right/down buttons being wrong were actually problems in SynthScrollBarUI, not in BasicScrollBarUI as originally evaluated.

The problem of the scroll buttons disappearing (not painting) when pressed is actually either an issue in skin.laf, or in SynthButtonUI (depending on how you look at it). It is my opinion that Synth states should *always* include either ENABLED or DISABLED in the state list. SynthButtonUI does not. If pressed, it uses PRESSED, not PRESSED | ENABLED as I would have expected.

skin.laf doesn't define a state for PRESSED, just for ENABLED and DISABLED. It probably should have a PRESSED state anyway, but this seems like a bug in synth as well.
                                     

EVALUATION

The first two issues (thumb not overlapping with buttons, and down, left, and right buttons not acting correctly on press) are both related to the overlapping button implementation in BasicScrollBarUI. The other issue (pressed state not painting correctly) is a Synth/Nimbus issue.