We should centralize WM_GETOBJECT OBJID_QUERYCLASSNAMEIDX tests in a test_WM_GETOBJECT() in misc.c instead of having them all over the place. For example, see test_WM_STYLECHANGED() and test_WM_THEMECHANGED(). The test can be easily done with a test array. Previous WM_GETOBJECT tests for other controls should be moved into test_WM_GETOBJECT() as well.
As for the header behavior difference between v5 and v6, I think it's best to wait for the comctl32 separation to be finished. I am working on that. Hopefully, it won't take too long to get upstream. Please see https://gitlab.winehq.org/zhiyi/wine/-/commits/comctl32-separation.