Fix left panel glow in Safari (#7236)

Add back removed CSS

res/css/views/elements/_ResizeHandle.scss

@@ -20,6 +20,15 @@ limitations under the License.
     z-index: 100;
 }
 
+// We'd like to remove this, but this makes matrixchat's resizehandle's
+// negative margin & greater than its positive padding. If it's the same
+// or less, Safari gets confused about overflows somehow and
+// https://github.com/vector-im/element-web/issues/19863 happens.
+.mx_MatrixChat > .mx_ResizeHandle.mx_ResizeHandle_horizontal {
+    margin: 0 -10px 0 0;
+    padding: 0 8px 0 0;
+}
+
 .mx_ResizeHandle.mx_ResizeHandle_horizontal {
     margin: 0 -5px;
     padding: 0 5px;