Update BoxPanel custom panel example

[michael-hawker]

Used this as an example for a template we're building in the Windows Community Toolkit and ran into some issues with the sample and code. Figured I'd contribute back to the article here the fixes I had to do.

Fixes:

• Off-by-one logic error for cutting size of box (for instance try '6' as a value, should be 2x3 not 3x3 still) (from my understanding of the scenario)
• Missing { in LimitUnboundedSize method
• Missing ; in ArrangeOverride method
• Improper definition of a Dependency Property (guidelines are strict here about property and DP names aligning otherwise issues can occur, picked a more standard XAML based Panel name here as well for this scenario.
  • Also added changed callback to invalidate layout so that the dependency property change actually updates the layout of the control immediately.
• Provided more context for where the adjustment to the code goes.

[PRMerger8]

@michael-hawker : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[Jak-MS]

@jwmsft

Can you review this PR?

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[jwmsft]

@michael-hawker, thank you!

@sign-off

[michael-hawker]

Thanks folks! Realized after there are some other optimizations around not using so many local variables in the example, but can do that as a separate thing later, if desired.

I know it's also probably to reduce the complexity of the code a bit, but at the same time I feel like it's good to promote best practices. Let me know if you'd like me to open another PR for that after.

[jwmsft]

Thanks folks! Realized after there are some other optimizations around not using so many local variables in the example, but can do that as a separate thing later, if desired.

I know it's also probably to reduce the complexity of the code a bit, but at the same time I feel like it's good to promote best practices. Let me know if you'd like me to open another PR for that after.

@michael-hawker, thanks for this. It's always a balance between keeping code simple for the docs and showing best practices. For a complete, complex example like this, I think it's okay to make it more complex and show the best practices. Please feel free to open another PR with your optimizations.