dillon1000/react

Warn about conflicting style values during updates (#14181)

This is one of the most insidious quirks of React DOM that people run into. Now we warn when we think an update is dangerous.

We still allow rendering `{background, backgroundSize}` with unchanging values, for example. But once you remove either one or change `background` (without changing `backgroundSize` at the same time), that's bad news. So we warn.

Fixes #6348.