LrSystemInfo.appWindowSize() returns incorrect values for height depending on Window > Screen Mode, making it impossible for plugins to know the maximum size of windows they can create. Since the SDK doesn't report the current screen mode, it's impossible for plugins to know the precise maximum size.
The following tables show the heights returned by appWindowSize() for the combinations of operating system, the operating-system display scaling, and LR's Screen Mode. "Useable window height" is the height of that part of the app window that SDK dialogs can occupy. Erroneous return values are in red.
Mac OS 10.14.3: RackMultipart20190311125765fdo-ab54fc2d-bb66-480d-895d-ae4fad303058-2027888312.png
Windows 10: RackMultipart201903112445727t0-9e0b4b82-43b9-4509-a50d-8c4339694c47-683099371.png
A related bug is that Windows appWindowSize() and displayInfo() report units in physical pixels, rather than the scaled pixels used by LrView. (The Mac SDK doesn't have the bug.) This makes it doubly hard for plugins to know the maximum size of windows they can create: