Windows Conda install won't launch

[thomashampson]

Describe the bug Conda install of mantidworkbench doesn't launch on SOME Windows machines:

qt.qpa.plugin: Could not find the Qt platform plugin "windows" in ""
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

DllOpen-[Error] Could not open library C:\Users\dsc49661\AppData\Local\mambaforge\envs\mantidnightlytest\Library\plugins\qt5\MantidScientificInterfacesIndirectQt5.dll: failed with error 126: The specified module could not be found.
DllOpen-[Error]
DllOpen-[Error]
qt.qpa.plugin: Could not find the Qt platform plugin "windows" in ""
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

The issue that is preventing it from launching is the qt platform plugin related error. The missing dll doesn't prevent launch.

I think the underlying cause is that qt6-main is being installed into the conda environment, which is being brought in by vtk. Since occt v7.8.1, the occt all build variant is being installed. On an older conda environment I have, the novtk is installed.

I think the solution would be to try to specify the novtk variant of occt.

To Reproduce Install 6.10.0rc1 smoke testing candidate and try to launch with workbench

Platform/Version (please complete the following information):

• OS: Windows
• Mantid Version 6.10.0rc1, also nightly version 6.9.20240619.1320

[thomashampson]

I think there are two separate issues here so I'll split one off.

[thomashampson]

split the dll issue into https://github.com/mantidproject/mantid/issues/37587

[thomashampson]

I'm not sure why this is only happening on some machines, but presumably we want to make sure that qt6-main is not installed.

[github-actions[bot]]

Closed by #37588.