Scrollbars always take up space, even when hidden

[bjorn]

The scrollbars of the ScrollView are hidden when horizontal scrolling isn't needed. However, they still take up space, which is unexpected.

Screenshot showing the problem when running SLINT_STYLE=fluent cargo run -p gallery, with the issue highlighted by red lines (the text of the left TextEdit clips too early at the bottom):

This issue is visible for fluent, material and cupertino styles. In native style, the scrollbars don't hide at all when they are not needed (but would be nice if they would).