What if the client sets the z-index for the whole splitter to 300? I think what needs to be done here is enforcing the whole splitter to be a stacking context (https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Understanding_z_index/The_stacking_context), so the z-index of `#draggable` becomes relative to the parent, not absolute. The mentioned link doesn't list any suitable ways to create a stacking context here, but there is a Chrome-specific (probably WebKit-specific) trick described in #1320 that can do that easily (a no-op `transform: scaleZ(1);` on `:host`). It's already used for a different purpose [here](https://github.com/dart-lang/spark/blob/master/widgets/lib/spark_suggest_box/spark_suggest_box.css#L27).