Linux/Wayland: QVulkanWindow in window container cannot be transparent

Steps to find bug:

1. Get hello Vulkan widget example
2. Edit clear alpha color value to less than 1.0
3. Set mainWindow attribute Qt::WA_TranslucentBackground

Exact changes:

Line 455 in trianglerender.cpp change to:

VkClearColorValue clearColor = {{ 0, 0, 0, 0.5 }};

Line ~108 in main.cpp add:

mainWindow.setAttribute(Qt::WA_TranslucentBackground);

After this is done the background should be at least semi-transparent.

But on Wayland the background remains completely opaque.

What I found:

• Xorg environment works fine
• Setting QT_QPA_PLATFORM=xcb variable "fixes" the problem
• GLFW-Wayland does not have this issue