Reports

I have improved @naren-murali's answer. I think this implementation is what you need.

.custom-hover
  .mat-expansion-panel:not(:has(.mat-content-hide-toggle)):hover
  .mat-expansion-panel-header::after,
.custom-hover
  .mat-expansion-panel:not(:has(.mat-content-hide-toggle))
  .mat-expansion-panel-header:focus::after {
  background-color: red;
  content: '';
  display: inline;
  width: 16px;
  height: 2px;
  margin: 15px -4px 0 -12px;
}

https://stackblitz.com/edit/p4jjwk-hkndbv?embed=1&file=src%2Fstyles.scss&view=preview

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