Currently CT_CONTROLS_GROUP is made slightly useless (even in basic scenarios) because it doesn't dynamically resize depending on visible content.
CT_CONTROLS_GROUP should automatically scale the internal width and height (and therefore the vertical and horizontal scrollbars) depending on what controls are visible and the size of the visible controls rather than the maximum size of the controls on creation. I can't find any way to manually force an update either).
That's intended behavior, controls group should not scale based on their contents. In many situations, you'll want it to remain static, while the content is scrollable (notable examples: diary text, field manual, functions viewer, welcome screen, etc.)
Since you are in control of its content, you can resize it manually (using https://community.bistudio.com/wiki/ctrlSetPosition), ideally applying some limits so it won't overflow outside of the screen.
I believe I explained a little incorrectly, I was referring to the scrollable overflow are rather than the actual controls group itself, but looking at those examples I see that the intended use is slightly different than I imagined. But I'll find a work around. Thank you for the reply.
The fact that onDrop doesn't fire on controls inside a controls group does need addressing though I think. I've made a mission and uploaded it to this bug report: