message: also apply sticky header bg color when backdrop-filter is not supported
diff: // set the user avatar (if any) as a background so
  // it can be blurred by the tag panel and room list
  -@supports (backdrop-filter: blur(100px)) {
  +@supports (backdrop-filter: none) {
  .mx_LeftPanel2 {
  background-image: var(--avatar-url);
  background-repeat: no-repeat;
  backdrop-filter: blur(175px);
  }
  - // sticky headers need their own blur
  -
  + // sticky headers need their own blur, and their own background to make it opaque
  .mx_RoomSublist2_headerContainer_sticky {
  background-size: cover;
  background-image: var(--avatar-url);
  background-position: center;
  }
  +}
  +/* outside of @supports as the backdrop-filter will be ignored, and we still apply the mostly-opaque background color */
  .mx_RoomSublist2_headerContainer_sticky .mx_RoomSublist2_stickyHeaderBlur {
  backdrop-filter: blur(100px);
  background-color: $roomlist2-bg-color;
  }
  -}
  .mx_RoomSublist2_showNButton {
  background-color: transparent !important;
  