While looking at https://bugreports.qt-project.org/browse/QTBUG-36292 it reminded me of something I've noticed before. X11 has the concept of the PRIMARY selection: the text that was last selected and is currently shown as selected. Normally it's only one piece of text. If you start up two terminal windows and select text in one, then select text in the other, the first selection isn't selected anymore because the second one has become the primary selection. But if you start up any Qt application and select some text, either in a widget or in QtQuick, then select some text in your terminal, the Qt application still has a selection. It should not. Probably the main reason for this behavior is that you can paste the text anywhere with the middle mouse button, so it needs to be unambiguous what you are going to paste. In fact it will paste the last-selected text; the Qt application doesn't provide the primary selection anymore, it just isn't telling the truth about that.

(The inverse case is already OK: if you select text in the terminal first, then when you select some text in the Qt application, the text in the terminal is no longer selected.)

I'm not sure how to fix it yet though.