[TreeView] no selected background color applied on mobile device

[tonyhallett]

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

Current Behavior 😯

There is no selected background color for a selected tree item.

Expected Behavior 🤔

There is selected background color for a selected tree item.

Steps to Reproduce 🕹

https://codesandbox.io/s/wizardly-maxwell-e7mmk

Steps:

1. Select item

https://github.com/mui-org/material-ui/blob/81c20b849f7aeddb4004ed215d80f7ca609354c1/packages/material-ui-lab/src/TreeItem/TreeItem.js#L32

export const styles = (theme) => ({
  /* Styles applied to the root element. */
  root: {
    listStyle: 'none',
    margin: 0,
    padding: 0,
    outline: 0,
    WebkitTapHighlightColor: 'transparent',
    '&:focus > $content $label': {
      backgroundColor: theme.palette.action.hover,
    },
    '&$selected > $content $label': {
      backgroundColor: fade(theme.palette.primary.main, theme.palette.action.selectedOpacity),
    },
    '&$selected > $content $label:hover, &$selected:focus > $content $label': {
      backgroundColor: fade(
        theme.palette.primary.main,
        theme.palette.action.selectedOpacity + theme.palette.action.hoverOpacity,
      ),
      // Reset on touch devices, it doesn't add specificity
      '@media (hover: none)': {
        backgroundColor: 'transparent',//******************************************
      },
    },
  },

[merterden98]

I just did a little investigating, and was wondering what the point of the media query was here. Removing it seems to restore your expected functionality, perhaps @oliviertassinari or @joshwooding could elaborate on this more.

[joshwooding]

Nice spot. Didn't notice it then but this was fixed in #21514

[oliviertassinari]

@joshwooding Awesome, it sounds like we should be able to roll out the approach of Pagination and TreeView to the other components, DatePicker's included :). I have opened an issue so we don't forget mui/material-ui-pickers#1943.