ci: X11 headers + xvfb on the Ubuntu lanes so the editor tests run

The end-to-end editor tests embed real windows; without X11 dev headers
they are skipped at configure time, and without a display they cannot
run. Install libx11/xrandr/xext/xcursor + xvfb and run the test steps
under xvfb-run on Linux.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: jcelerier

.github/workflows/build_cmake.yml

@@ -45,7 +45,8 @@ jobs:
                              chmod +x ./llvm.sh ; \
                              sudo ./llvm.sh 22 ; \
                              sudo apt update ; \
-                             sudo apt install ninja-build clang-22 lld-22 libc++-22-dev libc++abi-22-dev",
+                             sudo apt install ninja-build clang-22 lld-22 libc++-22-dev libc++abi-22-dev \
+                                              libx11-dev libxrandr-dev libxext-dev libxcursor-dev xvfb",
               sdk: "/opt/ossia-sdk-x86_64",
               pre_build: ""
             }
@@ -61,7 +62,8 @@ jobs:
                              sudo apt install lsb-release wget software-properties-common ; \
                              sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test ; \
                              sudo apt update ; \
-                             sudo apt install ninja-build gcc-16 g++-16 libgstreamer1.0-dev",
+                             sudo apt install ninja-build gcc-16 g++-16 libgstreamer1.0-dev \
+                                              libx11-dev libxrandr-dev libxext-dev libxcursor-dev xvfb",
               sdk: "/opt/ossia-sdk-x86_64",
               pre_build: ""
             }
@@ -164,7 +166,12 @@ jobs:
             export PATH="${{ matrix.config.path }}:$PATH"
           fi
 
-          cmake --build . --target test
+          # The editor tests open real windows: give them an X server.
+          if [[ "$RUNNER_OS" == "Linux" ]]; then
+            xvfb-run -a -s "-screen 0 1280x960x24" cmake --build . --target test
+          else
+            cmake --build . --target test
+          fi
 
       - name: Build release
         shell: bash
@@ -205,4 +212,9 @@ jobs:
             export PATH="${{ matrix.config.path }}:$PATH"
           fi
 
-          cmake --build . --target test
+          # The editor tests open real windows: give them an X server.
+          if [[ "$RUNNER_OS" == "Linux" ]]; then
+            xvfb-run -a -s "-screen 0 1280x960x24" cmake --build . --target test
+          else
+            cmake --build . --target test
+          fi