If you create a ScrollView that contains an NSView that intersects the scroll view's scrollbars, the scrollbars (or "scrollers") in that scroll view can no longer be clicked and dragged in those areas of intersection; clicks go directly to whatever the NSView inside the scroll view is.
This bug appears both with custom NSViewRepresentable-conformances and built-in SwiftUI views that wrap NSViews, like Button (which wraps NSButton.)
The two-finger touchpad scroll gesture works fine — it's clicking and dragging scrollbars that does not. The "Show scroll bars" preference must also set so that macOS only shows scrollbars when you're actively scrolling (i.e. via two-finger gesture). In other words, having that preference set to "Always" will prevent you from seeing this bug.
Filed as FB9757718.