Thanks for attaching the project - it made this easy to reproduce.
Here's what happened: the button inside your GroupBox has ended up spanning a 5 × 8 area of the GroupBox's grid at the Wide breakpoint (it's a normal 1 × 1 cell at the Compact and Medium breakpoints). Because that's far taller than the GroupBox itself, the resize handles along the button's bottom edge are drawn below the bottom of the designer canvas, so they can never be grabbed and dragging the top handles never makes the visible part smaller. So the button isn't locked, but I completely see why it looks that way.
You can fix the project right now:
- Select Button1 and make sure the Wide breakpoint is selected.
- In the Inspector, scroll down to the Placement section.
- Set Row Span to 1 and Column Span to 1.
The button returns to its normal size immediately.
I've logged the underlying designer behaviour as a bug: a control inside a container shouldn't be able to outgrow the container's grid in the first place, and its resize handles should always be reachable.
https://feedback.objo.dev/bug/1565