Just a quick correction post:
"When a tab becomes in active" should read "inactive"
And for my serious question:
"One of the quirks of the WM_DWMSENDICONICTHUMBNAIL message is that it passes the x- and y-coordinates backwards. Most window messages put the x-coordinate in the low word and the y-coordinate in the high word, but WM_DWMSENDICONICTHUMBNAIL does it the other way around"
Why does this one behave differently? Is there an amusing backstory to this?