Adwaita: make scrollbar handles more comfortable as a click target
Scrollbar handles in Adwaita are primarily intended to serve as an indicator when using a touchpad or a scroll wheel. However, they *can *be dragged, and in this situation they are on the narrow side, and can therefore feel a little stressful to target.
Making the handles slightly wider in the hover state could help with this. We could also reduce how much trough is shown on either side.