When scroll bar is expanded, clicking above or below the bar should scroll up or down

[dweymouth]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

Fyne's scroll bars do not support the standard behavior of clicking above or below the scroll bar itself, when it is "activated" (ie in its expanded state), to scroll up or down.

Is it possible to construct a solution with the existing API?

No response

Describe the solution you'd like to see.

Clicking above or below the scroll bar scrolls up or down.

[andydotxyz]

Could be useful.
I wonder if this also implies we should draw a background colour there to make it clear the taps belong to scroller and not the widgets that are visible under it?