I have several screens where items are hidden, timers are run automatically, etc. when the screen becomes visible. This can be very frustrating when trying to design the app as all of these things occur in edit mode and in preview.
I have a screen that uses a timer to navigate to another screen after a specific timeout. Working on the screen is made more difficult by the fact that I can't stop the timer easily and it can be a race against the clock (quite literally) to find the right property in time.
On several screens I have controls that are hidden by default. I have to change their visible state before I can work with them effectively.
For an extreme case, imagine a screen that is accidentally set to navigate immediately to another screen. It is literally impossible at this point to fix this problem in the designer.
In order to work with some screens I have to disable timers, show controls, etc. All of which needs to be reverted when saving the app or it will not function as intended.
We need some way to control these things so that we can design screens with dynamic content.
One option is to have settings within the designer that control certain aspects of the underlying code so that non-visible objects are displayed, timers do not start automatically and certain actions cannot be taken, such as blocking all navigation from the screen in the designer.