I have a user control with a minimum height and width. If I minimize Management Client so the user control does not fit the plugin panel any more, no scrollbars appear. How do I enable scrollbars for my plugin?
Please disregard. It was a WinForms issue. I was using MinimumSize, not AutoScrollMinSize, together with AutoScroll.
Start your UI control with a panel with AutoScroll is my best bet, you need to resort to basic WinForms methods in your controls because there is no automatic inheriting of scroll bars from the plugin UI elements.
I was mildly surprised that none of the plugin samples for the Management Client shows anything like this.
Panel, AutoScroll and WinForms tricks might be better found in other developer forums.
Just to help any other lost souls in the future, here is a more detailed explanation what I did. I have a standard UserControl with various textboxes, buttons and what not, which is used by an ItemManager and added to the site navigation tree, as per standard procedure. So the UserControl is displayed in the right information panel when clicking on one of my custom items. All elements in my UserControl use fixed positions and achoring for scaling. When using a low res monitor a scrollbar was needed, so I simply toggled AutoScroll to true and set MinimumSize on my UserControl and thought it would solve the problem. MinimumSize worked just fine, but no scrollbars appeared. I googled the problem, but solutions to WinForms issues are hard to find in 2023. I was so sure about MinimumSize so posted this question. Then I looked up what AutoScrollMinSize did. “The AutoScrollMinSize property is used to manage the screen size allocated to the automatic scroll bars.” …eh 
So I moved the values from MinimumSize to AutoScrollMinSize and scrolling worked as expected.