Changing size dynamically doesn't work in strict mode

[tmeasday]

Describe the bug

Dynamically changing the size has no effect in React strict mode, either programatically or via devtools.

To Reproduce

Steps to reproduce the behavior:

1. Go to https://codesandbox.io/s/048norw86p
2. Open the preview window fullscreen
3. Open React devtools
4. Change the size prop to some other value
5. Notice nothing happens

Expected behavior

Changing the size prop should change the size of the fixed pane.

Additional context

As the getDerivedStateFromProps function is state dependent (it state based on the previous state), in React's strict mode it gets tripped by this sort-of zany check:

https://github.com/facebook/react/blob/067cc24f55ef01a233be9e7564b337e35fed72ea/packages/react-reconciler/src/ReactFiberClassComponent.js#L139-L148

So when you change props, getDerivedStateFromProps gets called twice and the second time, this check returns false, so newState.paneXsize is not set:

react-split-pane/src/SplitPane.js

Lines 235 to 238 in ec8d4ae

	const shouldUpdateSize =
	props.size !== undefined
	? props.size !== instanceProps.size
	: defaultSizeChanged;

I could have this explanation wrong, I'll admit it's pretty confusing.

[tmeasday]

I've started trying to put a PR together to fix this but I have gotten stuck with StrictMode. Here is my progress:

https://github.com/tomkp/react-split-pane/compare/master...tmeasday:fix-309-updates-in-strict-mode?expand=1

Note this test passes if you remove the StrictMode (this is expected) but fails with the StrictMode, but not for the reason I am hoping. Instead what happens is the jsx renders to null and the test is unable to find the panel.

I have no idea why it would be that if X renders to something, <StrictMode>X</StrictMode> would render to null. Does anyone have any idea?

[ghost]

Even if I do not use StrictMode, dynamic sizing in controlled components is not working..

[tmeasday]

@kidkkr - do you have a reproduction? If I remove the StrictMode in my repro above it works, at least altering the values from devtools.

[ghost]

Sorry. I did misunderstand situation. https://codesandbox.io/s/o9ly13qovy it works well. 😃
It seems that we use StrictMode in our product.

[wuweiweiwu]

I haven't had much experience with StrictMode but Ill dig around.

[tmeasday]

OK, my PR now works: #315 cc @wuweiweiwu

[wuweiweiwu]

Awesome! I'll take a look ASAP @tmeasday

[wuweiweiwu]

Released in 0.1.83!!