[test] Remove position:relative from regression container

[aarongarciah]

Follow up on #43656 (comment)

Preview: https://deploy-preview-43743--material-ui.netlify.app/material-ui/react-slider/#music-player

[mui-bot]

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https://deploy-preview-43743--material-ui.netlify.app/

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Generated by 🚫 dangerJS against f3748bd

[aarongarciah]

The argos changes seem unrelated:

• https://app.argos-ci.com/mui/material-ui/builds/32056/108821862
• https://app.argos-ci.com/mui/material-ui/builds/32056/108821888
• https://app.argos-ci.com/mui/material-ui/builds/32056/108822015

[Janpot]

flakyness of https://app.argos-ci.com/mui/material-ui/builds/32056/108822015 is not solved 😢

[aarongarciah]

The argos changes seem unrelated:

• https://app.argos-ci.com/mui/material-ui/builds/32056/108821862
• https://app.argos-ci.com/mui/material-ui/builds/32056/108821888
• https://app.argos-ci.com/mui/material-ui/builds/32056/108822015

The first one is caused by the removal of position:relative.

[aarongarciah]

@Janpot I updated the MusicPlayerSlider demo to be self-contained. Now:

• It doesn't depend on some parent element setting position:relative.
• It includes the necessary padding to make it look good.

before: the gradient background covers all of the demo surface, including the demo container padding. It overflows the rounded corners of the demo container.

after: the demo is self contained, doesn't overflow the demo container. Not as beautiful as before. Ideally we should have a way of removing padding in specific demos.

[Janpot]

This is great

[oliviertassinari]

flakyness of https://app.argos-ci.com/mui/material-ui/builds/32056/108822015 is not solved 😢

@Janpot But we have a cleaner screenshot environment at least 😄. Issue created to propagate it: mui/mui-public#199

I updated the MusicPlayerSlider demo to be self-contained.

@aarongarciah Nice, I tried #43748 to push it a step further.