If you are running on any flavour of windows this is the expected behaviour. Nasty, isn't it. The work-around is to size the windows to fill the screen without putting them into the maximised state.
I see other posts now that relate to this issue. Some blame Windows as the culprit. As a longtime Windows developer in other environments, I can tell you that this isn't the case. Its a bug in FileMaker. Windows don't magically resize themselves in Windows unless your code tells them to.
I believe the issue is that FileMaker only allows 1 window to be Maximized. Opening another window will tell the new window to be Maximized because the parent window was. This will resize the parent window.
I get around this by haveing windows be sized when they open as part of the script and have the window be a few pixels smaller then maximum. If the window is just a little smaller then Max, it won't trigger the resizing you are seeing. Check out the Get functions that pertain to Screen. They will let you determine the screen size and come up with a value close to Maximum but just a bit less so that you don't have the resize issue.
Nope; if you tell it what size to make the new window, it still does it.
This is, and always has been the standard behaviour on Windows. It's not likely to change any time soon either. Your only option, as others have said, is to control the size of all of your windows. You'll find most people blame the Windows OS for the cause. Perhaps others can shed some light on the reasons.
Regardless, there is nothing you can do about it. Now that you're aware of this behaviour, you can implement the same workarounds the rest of us have been doing for years. It's not that hard.
The point is not the new window, rather the main window, which you would seek maximized state. Instead of maximizing the window, you need to set it's size precisely.
I name this main window and refer to it by name in my "FullWindow" script.
Try this, it works for me:
In your opening script give the main window a name of "Main" for instance.
Set Window Title[ name: "Main"]
Perform Script[ "FullWindow" ]
the FullWindow script
Move/Resize Window( Name: "Main"; Current file; Height: Get( WindowDesktopHeight ) - 10; Width: Get( WindowDesktopWidth ) - 12; Top: 0; Left: 0 )
Exit Script [ ]
I use a script so that any time I need to reinforce the main window I just call it.
Also it is easy to find and maintain any time a change is needed.