napari: 'Alt' modifier key not being detected when used alone on linux

🐛 Bug

The ‘Alt’ modifier key is not being detected in a mouse callback on a Linux system here, printing event.modifiers shows an empty tuple. This works as expected on my mac.

Interestingly, ‘Control’ + ‘Alt’ is working absolutely fine on the same linux box

To Reproduce

import napari

def callback(viewer, event):
    print(event.modifiers)

viewer = napari.Viewer()
viewer.mouse_drag_callbacks.append(callback)

napari.run()

Expected behavior

Environment

lg7-1g_vchandra> napari --info napari: 0.4.17 Platform: Linux-4.18.0-425.13.1.el8_7.x86_64-x86_64-with-glibc2.28 Python: 3.10.7 (main, Sep 9 2022, 12:08:13) [GCC 12.1.0] Qt: 5.15.2 PyQt5: 5.15.9 NumPy: 1.24.2 SciPy: 1.10.1 Dask: 2023.2.0 VisPy: 0.11.0 magicgui: 0.6.1 superqt: unknown in-n-out: 0.1.6 app-model: 0.1.1 npe2: 0.6.2

OpenGL:

• GL version: 4.6.0 NVIDIA 470.86
• MAX_TEXTURE_SIZE: 16384

Screens:

• screen 1: resolution 1920x1080, scale 1.0

Plugins:

• napari: 0.4.17 (77 contributions)
• napari-console: 0.0.7 (0 contributions)
• napari-svg: 0.1.6 (2 contributions)
• napari-threedee: 0.0.3 (18 contributions)

Additional context

reported by Vish Chandrasekaran @ichbintux