Fix WinUIBackend NavigationSplitView initial layout

[stackotter]

For some reason split views get laid out very incorrectly on the initial load, but snap to a correct layout as soon as some state changes or the window gets resized.

It's possible that WinUI isn't able to produce a correct size before the first render and gives us a dud value. But I haven't verified that suspicion.

You can reproduce this issue by running SplitExample on Windows with WinUIBackend (the default).