scroll-area(css): use opacity instead of visiblity

[MentalGear]

Currently, the scroll-area thumbs just disappear when not needed, which is rather sudden and draws user attention, even though it shouldn't. Please consider using instead opacity = 0/1 instead of switching visibility to fade them out softly.

bits-ui/packages/bits-ui/src/lib/bits/scroll-area/scroll-area.svelte.ts

Line 185 in 6e251ff

this.isVisible = false;