The 'PRIMARY' Clipboard (Third/Middle Mouse Button) Paste is Missing

Selecting text, then trying to paste it into text-areas/boxes (such as those in 'Create Issue' at 'bugreports.qt.io') using the middle mouse button does not paste the selected text.

The selection itself does move to the box/area, but nothing will be pasted.

Selected text does get copied and pasted outside the web area, such as the address bar or a different application (such as KWrite).

Unfortunately, I'm not sure at what point this started happening, but it was quite a while ago already, and it will take me some time before I would be able to bisect it out.

Tested using:

• Falkon 3.0.99 (e0d1175d)
• Qt WebEngine 5.12 (7de23e9f)
• LXQt >0.13.1 (git master)

Edit (2018-09-19):

Tested and confirmed using 'examples/webenginewidgets/simplebrowser' as well.