Pulled out from https://github.com/microsoft/playwright/pull/31900
I stumbled over `React.Children`, because it's the first time I saw that
used. https://react.dev/reference/react/Children lists `React.Children`
it as "Legacy" and mentions it's uncommon. Also, the fact that SplitView
only displays its first two children, and all others are silently
discarded, can be a surprise to some.
By separating things out into `sidebar` and `main`, not only do we give
the two elements names (otherwise one needs to remember that sidebar is
always the first child), but we also prevent any "third children" from
being dropped.
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