Try using xvfb-run: https://github.com/JuliaGraphics/Gtk.jl/blob/69c108069e9b3147841da7fb539dc9a4a0bedfc6/.github/workflows/ci.yml#L30. To be clear, you also need to actually pass prefix to the julia-actions/julia-runtest workflow: https://github.com/JuliaGraphics/Gtk.jl/blob/69c108069e9b3147841da7fb539dc9a4a0bedfc6/.github/workflows/ci.yml#L54-L56