plugin control embedded in ContainerControl macOS

A plugin using the cocoa NSBox, implemented according to a sample plugin project from Xojo (It uses the Constructor behavior to alloc the NSBox, and the HandleGetter to give Xojo the address of this NSView) shows a 2 times miniaturized NSBox when the container with the embedded NSBox is shown on Window1 in design time. This miniaturization also occurs with our LexingControl. Once the app runs, the miniaturization does not occur. Further, when the plugin control is placed on Window1, not embedded in a ContainerControl, the size of the control is not affected. So it is something specific to the ContainerControl, reducing the plugin control size in design time.

Are we missing something, given Xojo’s group box shows the correct size in design time when embedded in a container control?

I have this filed <> because could not figure a workaround. Have provided more info with This behavior also occurs with 2018r1, and may go all the way back further; we only tested with the 64-bit version of the Xojo-IDE.