Screenshot taken always from the main window backbuffer on MacOS/Metal

[alemuntoni]

I was trying to capture screenshots from multiple windows, and I encountered unexpected behavior on MacOS (using the Metal backend).

When I invoke bgfx::requestScreenShot(fbh, "screen");, it appears that the back buffer of the main window is always used, regardless of the value of fbh.

You can reproduce the issue by adding the following lines in the window loop of the 22-windows example:

if (m_frame % 300 == 0) {
    uint32_t cnt = m_frame / 300;
    std::string filename = "temp/frame_" + std::to_string(cnt) + "_" + std::to_string(ii);
    bgfx::requestScreenShot(m_fbh[ii], filename.c_str());
}

were m_frame is just a frame counter.

Modified example here and diff here.

The screenshot capturing works as expected on Windows (using DX11) and Linux (using Vulkan), but on MacOS, all the screenshots are taken from the main window.

This is the patch: screenshot.patch.zip