Rendering errors with ShaderEffect after hiding and reshowing a window

The core issue appears to be the cleanup done at https://code.qt.io/cgit/qt/qtdeclarative.git/tree/src/quick/scenegraph/qsgthreadedrenderloop.cpp?h=5.15.2#n554 – this
cleanup is deleting material types when hiding that are subsequently being
used when the window is reshown.

The attached diff adds logging to qtdeclarative confirming the above. It will
generate logs like:

querying 0x7f97b0cdea80 type 0x7f97b0c9eb30
deleting cached materials (0x7f97b0c9eb30)
querying 0x7f97b0cdea80 type 0x7f97b0c9eb30

You can see on the 3rd line that m_type still refers to a QSGMaterialType that
has been deleted at the 2nd line.

The attached example can be used to reproduce. Comments in the file
describe the reproduction.

Workaround can be to set the ShaderEffect vertex shader to empty and set it back when the Window is visible again:

onVisibleChanged: {
        if(window.visible === true) {
            effect2.fragmentShader = effect2.fragmentShaderString
        }
        else {
            effect2.fragmentShader = ""
        }
    }