I have just tried myself by adding your code to the "Zooming and Scrolling with Track Line (1)" sample code for testing. In my case, it works normally. After pressing F1, I can use the mouse to zoom in. The window does not change size.
For your case, is it possible your Form is also handling the F1 key? Your code is only previewing the key. It is also possible for the Form to preview the key or to handle the KeyUp or KeyDown events.
If the above still cannot solve the problem, is it possible to provide a simple example that can reproduce the problem, so I can try it and trouble-shoot the issue?