Controls without the Align property cannot be placed directly on the MDI form
You tried to double-click on a control to have it placed on the MDI form and the control doesn't support the standard property Align. A picture box is the only standard control that can be placed on an MDI form. When an MDI child form is maximized within the parent MDI form, it will not cover any picture boxes.
An example of good usage: Create new MDIform and double-click on picture box
An example of bad usage: Create new MDIform and double-click on text box