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chromium/third_party/blink/web_tests/scrollbars/resources/scrollable-iframe-customscrollbar-windowinactive.html 

<!DOCTYPE html>
<style>
::-webkit-scrollbar {
    width: 12px;
    height: 12px;
    background: rgba(255,0,0,0.8);
}
::-webkit-scrollbar:window-inactive {
    background: rgba(255,150,250,0.8);
}
iframe {
    width: 100px;
    height: 130px;
}
</style>
<iframe src="scrollable-subiframe-nowindowinactive.html"></iframe>