This is confirmed by the tests in test_scrollbar() and manual tests on XP. Even though scrollbar
arrows and thumb are in fact transparent, DrawThemeParentBackground() is not called to paint the
parent background, leaving the transparent area untouched. On Windows, even if the scrollbar arrows
are reported to be transparent, the bitmaps for arrow parts in the stock themes are in fact opaque.

Fix Ice Cream Calculator slow scrolling after 4cb229a9 because the WM_PRINTCLIENT handling in the
application is slow.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=53337