After checking the code for a few days, I've realized it would be too much effort to change this behavior. The implementation is currently a mess, it will be difficult to change everything in small patches, so I will postpone this and come back at some point later (hopefully). For who might be interested, the issue is around ScrollbarTheme::nativeTheme, which returns a static pointer to a class that mostly have static methods, and doesn't keep track of what page/frame the scrollbar is inserted, neither scrollbars themselves keep track of it, and also not all of these methods receives a scrollbar as parameter, so we can't just put a page/frame reference inside scrollbar, for example (and this was one of my ideas). There's currently an implementation of what would be a custom scrollbar. To create it you must use RenderScrollbar::createScrollbar, but I couldn't make it work properly - I tried to force it to be used, but the resulting scrollbars geometry were bogus.

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