flasher777,
Details of that flag:
Choose the graphics backend for ANGLE. The OpenGL backend is soon to be deprecated on Mac, and may contain driver bugs that are not planned to be fixed. The Metal backend is still experimental, and may contain bugs that are still being worked on. The Metal backend should be more performant, but may still be behind the OpenGL backend until fully released. – Mac
Related article [09/24/2018]: “The end of OpenGL and OpenCL on the Mac” at:
“https://arstechnica.com/features/2018/09/macos-10-14-mojave-the-ars-technica-review/12/”