Add support of merging classes in withStyles

[synaptiko]

Hello,

we just migrated to tss-react in MUIv4 based project and we found a small difference in how withStyles work in MUI and here.

The difference is in maybe a special case. Here's a small example:

import InputBase from '@material-ui/core/InputBase';
import { withStyles } from 'common-ui/styles';

export const CustomInputBase = withStyles(
  InputBase,
  (theme) =>
    ({
      input: {
        backgroundColor: theme.palette.grey[50],
      },
    } as const)
);

const StyledInput = withStyles(CustomInputBase, () => ({
  input: {
     backgroundColor: 'red'
  },
}));

The problem is that StyledInput won't get red color as one would expect. The reason is that withStyles currently doesn't merge classes passed in props of the wrapping component.

I tried to add it by using mergeClasses function where I noticed another small issue. The problem is when classes doesn't contain all the keys; then the key present on classesOv would be missing in the output classes object.

[garronej]

Hi, thank you very much for that PR.
Sorry I didn't manage to make time to test it yet although it looks good!
I published a beta version incorporating your changes so you are not blocked until it make it to main.
3.3.2-beta.0.

[synaptiko]

@garronej Thanks for the quick reaction, I just upgraded to the beta version and it seems to work for our use-case.

[garronej]

I'm testing the changes right now.
I happy you're choosing to use TSS not just only out of necessity to upgrade to MUIv5. 😄

[garronej]

Hi @synaptiko,
Good catch on this bug, it was a pretty big one.
I got confused at first because the sniped you provided works as expected (if it's not the case for you, you might have a problem with your configuration).
Anyway I wrote another test and polished a little bit your PR that was otherwise very good.

Thanks for your contribution,

[synaptiko]

I got confused at first because the sniped you provided works as expected

I extracted it from our code and simplified it for this PR but I think I did a mistake there.

Thanks for the test, I wanted to provide it too but wasn't sure what's the right process.

[garronej]

No problem, you submitted a clean PR when you could just have opened an issue.