Indicate when sidebar content is scrollable

[twschiller]

Context

• When putting together a sidebar extension for the playground, depending on the user's screen height, they might not see any hints that there's more content below the fold in the sidebar
• https://pixiebrix.slack.com/archives/C02RAMRDF0R/p1659998686767219?thread_ts=1659997604.231749&cid=C02RAMRDF0R

Discussion

• Scrollbar visibility behavior is an OS-level setting. We've encountered the same issue on the Add Bricks modal and blueprint screen

General Preferences on Mac