1.) Mainly it is a box and therefore can only host one child. That is the reason why one control is replace by the other when adding a second control.

The GroupBox provides the facility to show content as a group, i.e. with border and title. However, the group box does not decide about the layout of the children, therefore a multi-child container is required. Otherwise the code for grouping (border and tirle) would have to be duplicate for HBox, VBox, Canvas, etc.

This concept of having a Box or Frame with additional functionality occurs often. For example the GuiRangeSelectionFrame allows to select a range of gui elements (controls, text etc.), however it is a single-child container and only allows on child. This usually is a multi-child container like HBox, VBox or Canvas.

Thereby, there is a clear separation between the additional functionality (provided by a Frame or Box) and the layout of multiple children (provided by HBox, VBox, Canvas). Thus, it is easy to combine these two aspects.

2.) Again, Dialog is a single-child container. You have to add first a container that defines the layout (HBox, VBox, Canvas) to the dialog and then add the controls to it.

@Curl: It might be a good idea to mark single-child and multi-child containers differently.

Friedger