How May I remember a selection?
I want to let the user choose some objects and then move
them to another Point by just double-clicking the background.
Since doubleclicking background loses actual selection
I got no selection anymore. Or Do I have to remember
the objects that were in the selection!?
Is the problem that you still want to support a single-click in the background causing the selection to be cleared? And that a single-click is always a “prefix” to a double-click?
If so, then you cannot do what you want in a mode-less fashion. Instead you must define your own Tool that is invoked in a modal fashion (presumably by some command). This tool would just handle DoMouseUp calls by calling GoTool.DoSelect, until a double-click comes along, when it would call GoView.MoveSelection and GoTool.StopTool.
Demo1 has a few custom Tools that you can learn from.