XCA menu bar items highlight but submenus don’t appear when running under RealVNC (LXDE); works on HDMI

[HeinzRuetschi]

Summary

When XCA is started inside a RealVNC session on the Raspberry Pi (Debian 13 + LXDE), the menu bar shows File/Extra/Help, but clicking them only highlights the top-level item; the submenu popup never appears. The same XCA version works correctly when run on the Pi with a physical HDMI monitor + keyboard.
Environment

XCA: 2.8.0
OS: Debian GNU/Linux 13 (Linux 6.18.34+rpt), Raspberry Pi 5
Desktop: LXDE
RealVNC Server: 7.13.1 (r57) on the Pi
RealVNC Viewer: 7.15.1 (r18) on Windows 10

Installation

Installed via: Pi menu → Preferences → Add/Remove Software → xca
Reproduction steps (RealVNC session)

Connect to the Pi using RealVNC Viewer (Windows 10).
On LXDE desktop, open XCA via Pi menu → Accessories → XCA.
In XCA, click the menu bar items: File (also occurs with Extra/Help).

Expected result

Submenus (File/Extra/Help) appear and submenu items are selectable/clickable.
Actual result

The clicked top-level menu item becomes highlighted/selected, but no submenu/popup items appear.
Control

Running XCA with the same Pi using a physical HDMI monitor + keyboard: submenus appear and XCA works normally.
Notes / Attempts

Tried running XCA with different Qt theme env vars (e.g., unsetting QT_QPA_PLATFORMTHEME); menu issue persists in RealVNC session.