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 4cb229a because the WM_PRINTCLIENT handling in the application is slow.
Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=53337
-- v2: comctl32/datetime: Do not paint parent background when painting arrows. uxtheme: Do not draw parent background even if scrollbar arrows and thumb are transparent. uxtheme: Use the brush from WM_CTLCOLORSCROLLBAR to paint SB_CTL scrollbar tracks if present. comctl32/tests: Add themed scrollbar background tests. uxtheme/tests: Add theme tests.