It's a real shame that platform native widgets had to be sacrificed for this to work. The easy way out was to switch to arbitrary app-drawn widgets across all platforms (which Firefox did for all basic DOM HTML elements); the "we'll do one better" alternative is to use app-drawn imitations of the system-native widgets (a la Qt) which Firefox is now doing for the scrollbar. But no one ever gets these widgets right (unnatural scrolling, non-identical behavior when clicking in the gapped region, different scrollable-to-nonscrollable widget ratios, colors not respecting certain subtle theming choices, etc, etc.

I wonder if there was an alternative specifically for the scrollbar here - some way of obtaining an outer "shell" (via win32k, but then basically "orphaning" it so that you can't do anything besides kill it when you're finished) that just provides the window with an empty scrollable element that is then populated by the restricted process.

That ship has really sailed though; I think the days of native widgets are quickly coming to an end.