right panel scroll bar improvements

[shcheklein]

Follow up after #1198

• When scroll is enabled and some items are not visible as we scroll the main page we want for the right bar to follow (highlighted item should be always visible).
• Safari - it's very easy (by scrolling the main page) to get it into the state when it completely ignores scroll inputs. Even if you hover mouse over it. right panel scroll bar improvements #1333 (comment)
• FF: scroll is not visible
• doc engine: fix scroll bar for the content sidebar #1284 (comment) - see screenshot, scroll bar overlaps with text

[jorgeorpinel]

When scroll is enabled and some items are not visible as we scroll the main page we want for the right bar to follow (highlighted item should be always visible).

➕ 1. I've been noticing this recently quite a bit. Thanks

[shcheklein]

An idea how to avoid forcing a scroll bar and still make it visible that there are some hidden elements there:

a slight delimter + shadow

[shcheklein]

And no shadow and the top delimiter when list fits 100% onto the screen:

[shcheklein]

Looks like Safari issue is happening only on the old versions < 14 (and/or old versions of macOS). It looks very minor and can be disregarded.

[julieg18]

FF: scroll is not visible

If I'm understanding correctly, it looks like you can see the right panel scroll bar in firefox: