Regarding the behavior after TTMSFNCPlanner.OnItemDblClick

I'm thinking of generating an edit form that uses AItem information within TTMSFNCPlanner.OnItemDblClick.

If you generate a Form in the above event, the Planner will scroll while the mouse focus remains on the TMSFNC Planner.

Is there any way to solve it?

Can you please give more details. When we test this here with:

procedure TForm1.TMSFNCPlanner1ItemDblClick(Sender: TObject;
  AItem: TTMSFNCPlannerItem);
  frm: TForm3;
  frm := TForm3.Create(self);

we do not see an issue.

The environment uses FNC Planner on Windows.

After doing some data processing in the generated frm provided in the sample, do

This is also an exaggerated example, but when I verified the test code, it will be reproduced if it is the following code.

begin begin
frm: = TForm3.Create (self);
Sleep (10000);
Application.ProcessMessages; <--- This kind of processing may be included.

Is it wrong to use Application.ProcessMessages etc. in the generated Form?

I would not recommend such construction.
What I think happens is that sleep() blocks the Windows message queue. But when you keep moving the mouse, the mouse messages keep entering the Windows message queue. When you do Application.ProcessMessages, the messages in the queue are processed again, but you do this before the new form is shown, so the messages from the queue go first to the form that is active before the new form is shown. I think this is unrelated to the TTMSFNCPlanner and would happen to any other VCL UI control.

Sleep is just a sample code, but I was able to recognize it as a code that I did not recommend after verifying it.
Currently, we are reviewing the processing when double-clicking.

Thank you