[Linux] Qt WebEngine: Long-press on touchscreen fails to open context menu in PDF viewer

Steps to reproduce

1. Launch the WebEngine Widgets Simple Browser Example or the WebEngine Quick Nano Browser on a device with a touchscreen
2. On the home page (chrome://qt), long-press the screen to bring up the context menu
3. Navigate to a PDF document (such as https://www.learningcontainer.com/wp-content/uploads/2019/09/sample-pdf-download-10-mb.pdf), then long-press the screen to bring up the context menu
    

Outcomes

• (Step #2) The context menu appears (Expected)
• (Step #3, Windows 10 22H2) The context menu appears (Expected)
• (Step #3, Ubuntu 22.04) The context menu does NOT appear (Not Expected)

Consequences
On a device with a touchscreen but no keyboard+mouse, users can use the PDF Viewer to enter Fullscreen mode (see reproducer at QTBUG-136988) but they will get stuck there, unable to open a context menu to exit Fullscreen mode