First unselected vertical (west) tab button renders incorrectly on macOS

The new mechanism for drawing tab buttons in the macOS style in Qt 5.12 doesn't work for the specific case of the top unselected tab button in west oriented tab bars. It draws the popup button at the wrong location, so the button doesn't get the right background color and shows part of the popup button control. Screenshot, sample code, and a patch to fix the issue are attached.