That is a possibility, you will note in my original query I suggested I started development on my MacBookPro ... I suspect I upgraded since then.
Also, a colleague of mine I sent the project to had no problems, also MacBookPro, don't have his machine specs on hand.
Thanks for the consideration. The original is quite a complex composition, lots of subcompositions etc. But I have stripped it all down to the attached. If you connect the image directly to the make unlit shader node, it works. If the GLSL shader is used, it doesn't. There are two windows grabs, correct_macmini.png and incorrect_macbookpro.png. I do not expect you to debug the GLSL shader but I'm curious/concerned as to why it works on the Mac Mini and on lots of other Macs but not my MacBook Pro.
The composition involves texture mapping onto OBJ read objects and image mapping for the texture through a GLSL shader. On the MacMini all is perfect, on the MacBook Pro it looks like the texture mapping is getting messed up.