I have seen cases where during a rebase (two nonModelItems) all entries in
viewIndicesByModelIndex beyond the second nonModelItem were off by 4 rather than
2 as I would expect. The only explanation I have for this is that the function
was called concurrently.
Improve this by working on a local variable and only assign to self at the end;
this is not a real fix for the concurrency issue of course, but it makes it much
less likely to be a problem in practice.
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