I got the the same error when popuping a form with the border set as fbDiaglogSizeable. I also tested MultiForm demo. The multiform demo is bit different since inside pupup form, in my case I created another form hosted on the first page sheet of pupup form.
So basically, this is code I used to test popup form:
newform := TForm3.CreateNew(procedure(AForm: TObject)
newform.Caption := 'Child form';
newform.Popup := true;
newform.Border := fbDialogSizeable;
Popup works, however, if I tried to resize the form by dragging the right-bottom corner, it shows an error saying "ResizeObserver loop limit exceeded" as you can see from pitcture I uploaded.
Another issue is : I checked the akRight (from Anchors) controls inside the popup form, to make sure they can maintain current position relative to an edge of its parent. However, if I resize the form, I did not see expected behavior of those controls, I am not sure if this issue is associated with above error...
Thanks, for your support in advance. To make it clear, I also have included my test file here test.zip (1.4 MB) for your reference. Any suggestion would be very helpful.
All the best,