lgtm, cc @spicyj

This should be added to htmlpropertyconfig, not here. @jimfb can you follow up with the details and confirm the testing is done right (look at other PRs for my comments about testing property vs attribute)

Thanks @zpao, I hadn't gotten a chance to look.

It seems like this new attribute should be covered by existing generic tests i.e. renderers/dom/shared/__tests__/ReactDOMComponent-test.

But since this attribute only has two legal values (hard and soft) and is specific to textareas, would it make sense to instead add this as a sort of pass-through property that has to meet the aforementioned invariants before it is set as a DOM attribute?

No, it's fine to just add it to the list of supported properties and not add a unit test, but you should test manually:

Are you able to make a local build and test that this property works correctly? It's important that both the initial render and updating the property work as those exercise different codepaths.

Yep, just built successfully and used it to both initially render and update the attribute. I can post the snippet if you want.

It looks like this field can be either a property or an attribute (eg. https://msdn.microsoft.com/en-us/library/ms535152(v=vs.85).aspx), so should we be using MUST_USE_ATTRIBUTE or null here or does it not matter @spicyj ?

If both work for updates, null is best.

@jas14 Let's use null unless there is a browser that requires it to be an attribute. Also, we need to rebase due to merge conflict. Otherwise, looks good to go.

Cool. Can you squash these to a single commit? Then we can merge.

Thanks!