b90818ec53
Fixes #145592. When slivers in a CustomScrollView overlap, this makes it possible to control which of the slivers will sit in front of the other one, meaning that it paints last and gets hit-tested first. In this version there are two choices: the first sliver paints in front, or the last sliver paints in front. The first sliver in front is the default; it's the behavior needed for SliverAppBar, or other situations where the first sliver is some sort of header. The existing behavior is actually quite a bit more complicated than that, with the "center" sliver painting in front, plus it varies depending on `shrinkWrap`. As far as I can tell this behavior isn't documented anywhere. It's equivalent to first-sliver-in-front whenever `CustomScrollView.center` isn't being used; and in fact there don't seem to be any tests in the tree that notice the difference between the existing behavior and first-sliver-in-front. So although I actually have an implementation that preserves the more complex existing defaults, I'm hopeful we won't need it: if neither google3 nor customer tests are relying on that undocumented distinction either, then we can go ahead and simplify this behavior without a breaking change.
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