Move backdrop filter to a canvas based solution

res/css/structures/_LeftPanel.scss

@@ -43,7 +43,6 @@ $roomListCollapsedWidth: 68px;
     // Note: The 'room list' in this context is actually everything that isn't the tag
     // panel, such as the menu options, breadcrumbs, filtering, etc
     .mx_LeftPanel_roomListContainer {
-        background-color: $roomlist-bg-color;
         flex: 1 0 0;
         min-width: 0;
         // Create another flexbox (this time a column) for the room list components

res/css/structures/_MatrixChat.scss

@@ -35,6 +35,16 @@ limitations under the License.
     height: 100%;
 }
 
+.mx_BackdropPanel {
+    position: absolute;
+    top: 0;
+    left: 0;
+    width: 100%;
+    min-height: 100%;
+    z-index: 0;
+    opacity: .15;
+}
+
 .mx_MatrixToolbar {
     order: 1;
 

res/css/structures/_SpacePanel.scss

@@ -27,6 +27,7 @@ $activeBorderColor: $secondary-fg-color;
     background-color: $groupFilterPanel-bg-color;
     padding: 0;
     margin: 0;
+    position: relative;
 
     // Create another flexbox so the Panel fills the container
     display: flex;