The Attachment Dialog

By default FORM objects acquire a size suitable to accommodate all children at their default size. RowForm and ColForm also perform child position and resize, depending on the scalability and attachments of children. The PosForm container simply sizes itself to accommodate all children.

While the SHELL container acts as a RowForm.
 
MGui Designer allows setting both object scalability and attachments.

By default MGui objects are always attached in all four directions to the container and the adjacent brothers. Non scalable objects are by default centered in a virtual area to be assigned to the object if it would resize.

By removing an attachment in one direction (say Left), the scalability property of the selected object in that direction won’t apply and the corresponding object size will remain to its default. For example, if you disable the Left attachment of a Label (which is horizontally scalable by default), the object width will be determined by the Label’s text (and font).

The dialog above also lets you control the alignment policy to adopt when a non scalable object should be placed in an area larger then the object’s size.

The Attachment Dialog slightly differs when the container of the selected object is a TableForm.
 
In this case, in fact, each child object is placed in a cell boundary so you cannot alter its attachments. 

You can however set the ‘Enlarge Alignment’ and, in addition, apply the selected alignment to the entire row (column) which the selected object belongs to.