To reproduce:
- Create a MDI application with a RadDock in the mainform.
- Add docked to the left ToolWindow to the dock.
- On a button click add a child form with a RadDateTimePicker in it.
- Set the picker Dock property to Top.

Workaround: 
- Use the following SizeChanged event handler:
 private void RadForm1_SizeChanged(object sender, EventArgs e)
{
    this.rdMain.Width++;
    this.rdMain.Width--;          
}