Support list collapsing and jumping

Fixes https://github.com/vector-im/riot-web/issues/14036

res/css/views/rooms/_RoomSublist2.scss

@@ -138,6 +138,34 @@ limitations under the License.
             text-overflow: ellipsis;
             overflow: hidden;
             white-space: nowrap;
+
+            .mx_RoomSublist2_collapseBtn {
+                display: inline-block;
+                position: relative;
+
+                // Default hidden
+                visibility: hidden;
+                width: 0;
+                height: 0;
+
+                &::before {
+                    content: '';
+                    width: 12px;
+                    height: 12px;
+                    position: absolute;
+                    top: 1px;
+                    left: 1px;
+                    mask-position: center;
+                    mask-size: contain;
+                    mask-repeat: no-repeat;
+                    background: $primary-fg-color;
+                    mask-image: url('$(res)/img/feather-customised/chevron-down.svg');
+                }
+
+                &.mx_RoomSublist2_collapseBtn_collapsed::before {
+                    mask-image: url('$(res)/img/feather-customised/chevron-right.svg');
+                }
+            }
         }
     }
 
@@ -251,6 +279,17 @@ limitations under the License.
                 background-color: $roomlist2-button-bg-color;
             }
         }
+
+        .mx_RoomSublist2_headerContainer {
+            .mx_RoomSublist2_headerText {
+                .mx_RoomSublist2_collapseBtn {
+                    visibility: visible;
+                    width: 12px;
+                    height: 12px;
+                    margin-right: 4px;
+                }
+            }
+        }
     }
 
     &.mx_RoomSublist2_minimized {

res/img/feather-customised/chevron-right.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="feather feather-chevron-right"><polyline points="9 18 15 12 9 6"></polyline></svg>