X11: popups such as QtQuick.Controls Combobox cause assertion failure when used on a touchscreen

This is another aspect of the popup event forwarding problem: when you touch a ComboBox, the popup is another window, so the button part of the combobox doesn't receive the TouchEnd event. (Without releasing your finger, dragging to select an item on the popup doesn't work correctly either.) But let's say you somehow manage to select an item; then it will crash the next time you tap on the combobox:

ASSERT: "itemForTouchPointId.isEmpty()" in file /home/rutledge/dev/qt5-stable/qtdeclarative/src/quick/items/qquickwindow.cpp, line 1793

The old touchpoint which caused the popup the first time would have been removed if the button had received the TouchEnd, but it didn't.