createMuiTheme overrides not working since 4.9.6

createMuiTheme overrides not working since 4.9.6

Since https://github.com/mui-org/material-ui/issues/20095, the isPlainObject has been modified. Testing constructorproperty is not multi-frame proof. This makes deepmerge do not merge incomming properties from given source when source is an Object created in another js context like another document iframe.

• [X ] The issue is present in the latest release.
• [ X] I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

The generated jss style does not conatin overrides, so desired style is not applied to components.

Expected Behavior 🤔

The generated jss style must contain overrides in order to have the desired styles applied to components.

Tech	Version
Material-UI	v4.1.11
React	16.14
Browser	all
TypeScript	no
etc.

[oliviertassinari]

Why change it in this particular instance?

@eps1lon I’m asking because it doesn’t seem to be something we fundamentally prevent from supporting. As far as I know, It’s not blocking. Developers can bypass the deepmerge method, by bypassing the createMuiTheme() method. Hence why I’m eager to have a reproduction to better understand the use case.

We know that multi-frame support is rarely used but we still support it. I suspect this specific case is unique and nonblocking. A drop in the bucket most developers would rather have us ignore (not pay the cost for it). Not every bug deserved to be fixed. I think that it’s a big factor in the “write your own” popularity, it allows different tradeoffs (but also come with ignoring problems you don’t know you have).

[fbaeta]

https://github.com/mui-org/material-ui/pull/23740#issuecomment-734486617

https://github.com/jonschlinkert/is-plain-object might also be a source of inspiration. My main concern is about adding bundle size for edge case almost nobody needs. You can modify the theme outside of the createMuiTheme() method.

Before all, thanks for your fast reply!

I understand that edge cases do not justify increasing bundle size 👍.

I’ve created a PR just to show the case, the goal was not really o be merged 😉 (the isPlainObject code is borrowed from lodash). I wanted to point out that:

• Using constructor to test if it’s a plain object is not the more resilient technique and should be reconsidered. If you don’t like lodash implementation, as you said, you might find other techniques. This change was introduced here https://github.com/mui-org/material-ui/issues/20095, so if in v5 you are planning to remove support of this popper version, you sould reconsider (again) to not use constructor technique.
• The isPlainObject is poorly tested.

In my current case, I CAN NOT upgrade material-ui up to 4.9.5 for now.