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

Steps to reproduce

1. Follow steps #1 to #5 of the reproducer at QTBUG-136988 to trigger Fullscreen mode
2. Long-press the screen to bring up the context menu, and select "Exit Fullscreen"

Outcomes

1. (Windows 10 22H2) The context menu appears, and we can exit fullscreen mode (Expected)
2. (Ubuntu 22.04) The context menu does NOT appear, so we cannot exit fullscreen mode via the touchscreen (Not Expected). The 'Esc' key does work as expected though.