Reuse server data if the client load is invalidated

[mquandalle]

Fixes #6459

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[mquandalle]

I think this issue was caused by #6309. So far I couldn't come up with an implementation that would work with both test cases, but @dummdidumm you probably have a better understanding than me of the logic here.

[mquandalle]

I realize that changed.url is not the perfect condition, because we will want to reuse the server data of a layout load even if the url changes, but I'm not familiar enough with the code base to determine the correct condition.