I can confirm that this problem exists for DECORATED, EXTENDED, UNIFIED, and UTILITY stage styles. It does seem to be more noticeable with EXTENDED, but it's still sometimes noticeable for the other styles. It doesn't seem to appear for UNDECORATED and TRANSPARENT stage styles.
This seems to be a preexisting problem: https://bugs.openjdk.org/browse/JDK-8088179 However, the fix seems to have been reverted later: https://bugs.openjdk.org/browse/JDK-8171852 Firefox seems to have had the same problem: https://bugzilla.mozilla.org/show_bug.cgi?id=1824053 They solved the problem by setting the DWMWA_CLOAK flag on the window, then painting the window with the initial color, and then uncloaking the window once done. However, they later reverted that fix because they noticed that cloaking the window also hides its opening animation: https://bugzilla.mozilla.org/show_bug.cgi?id=1906358 In the last bug report, they say that the white flash is caused by redirection bitmaps (which might be initially white?), and disabling them with WS_EX_NOREDIRECTIONBITMAP might solve the problem. However, a window without redirection bitmap would need to use DirectComposition to actually show its content. We can't easily use DirectComposition with D3D9, since it requires D3D11 (there's a way to do it still, because resources can be shared between D3D9 and D3D11). The DComp fix might also be implemented for the D3D12 pipeline only, once that lands.
