ci: prevent test-only PRs from bypassing cmuxTests compilation

.github/workflows/ci.yml

@@ -1,21 +1,8 @@
 name: CI
 
 on:
-  # Keep the full suite available for explicit validation, while always
-  # Changes to CI guard scripts also retrigger the full workflow through this path.
-  # running the lightweight GhosttyKit provenance check when its pointer,
-  # checksum, or helper changes in a PR.
   pull_request:
-    paths:
-      - ghostty
-      - scripts/download-prebuilt-ghosttykit.sh
-      - scripts/validate-xcframework-archive.py
-      - scripts/ghosttykit-checksums.txt
-      - tests/test_ci_ghosttykit_release_check.sh
-      - Resources/bin/cmux-hermes-agent-wrapper
-      - tests/test_hermes_wrapper_hooks.py
-      - tests/test_ci_change_areas.py
-      - .github/workflows/ci.yml
+    types: [opened, reopened, synchronize, ready_for_review]
   workflow_dispatch:
 
 permissions:
@@ -708,6 +695,21 @@ jobs:
             sleep $((attempt * 5))
           done
 
+      - name: Compile cmuxTests target
+        if: ${{ matrix.shard == 1 }}
+        run: |
+          set -euo pipefail
+          xcodebuild \
+            -project cmux.xcodeproj \
+            -scheme cmux-unit \
+            -configuration Debug \
+            -derivedDataPath "$CMUX_DERIVED_DATA_PATH" \
+            -clonedSourcePackagesDirPath "$PWD/.ci-source-packages" \
+            -disableAutomaticPackageResolution \
+            -destination "platform=macOS" \
+            "SWIFT_ACTIVE_COMPILATION_CONDITIONS=\$(inherited) CMUX_CI_APP_HOST_ISOLATION_REQUIRED" \
+            build-for-testing
+
       - name: Run Ghostty split-theme appearance regression
         if: ${{ matrix.shard == fromJSON(env.CMUX_APP_HOST_FOCUSED_REGRESSION_SHARD) }}
         run: |

tests/test_ci_change_areas.py

@@ -128,6 +128,10 @@ def test_app_source_runs_macos() -> None:
     assert_areas(["Sources/AppDelegate.swift"], macos=True, web=False)
 
 
+def test_test_only_changes_run_macos() -> None:
+    assert_areas(["cmuxTests/SurfaceCatalogTests.swift"], macos=True, web=False)
+
+
 def test_workflow_changes_run_everything() -> None:
     assert_areas(
         [".github/workflows/ci.yml"],
@@ -212,6 +216,49 @@ def run_linux_preflight(needs: dict[str, object]) -> subprocess.CompletedProcess
     )
 
 
+def test_compile_gate_builds_entire_test_target_and_propagates_failure() -> None:
+    script = workflow_job_step_script("app-host-unit-tests", "Compile cmuxTests target")
+    with tempfile.TemporaryDirectory() as temp_dir:
+        root = Path(temp_dir)
+        fake_bin = root / "bin"
+        fake_bin.mkdir()
+        arguments_file = root / "arguments"
+        compiler = fake_bin / "xcodebuild"
+        compiler.write_text(
+            '#!/bin/bash\nprintf "%s\\n" "$@" > "$COMPILER_ARGUMENTS"\n'
+            'exit "$COMPILER_EXIT_CODE"\n',
+            encoding="utf-8",
+        )
+        compiler.chmod(0o755)
+        for exit_code in (0, 65):
+            result = subprocess.run(
+                ["bash", "-c", script],
+                cwd=root,
+                env={
+                    **os.environ,
+                    "PATH": f"{fake_bin}:{os.environ['PATH']}",
+                    "CMUX_DERIVED_DATA_PATH": str(root / "derived data"),
+                    "COMPILER_ARGUMENTS": str(arguments_file),
+                    "COMPILER_EXIT_CODE": str(exit_code),
+                },
+                text=True,
+                capture_output=True,
+            )
+            assert result.returncode == exit_code, (result.stdout, result.stderr)
+            arguments = arguments_file.read_text(encoding="utf-8").splitlines()
+            assert arguments == [
+                "-project", "cmux.xcodeproj",
+                "-scheme", "cmux-unit",
+                "-configuration", "Debug",
+                "-derivedDataPath", str(root / "derived data"),
+                "-clonedSourcePackagesDirPath", str(root.resolve() / ".ci-source-packages"),
+                "-disableAutomaticPackageResolution",
+                "-destination", "platform=macOS",
+                "SWIFT_ACTIVE_COMPILATION_CONDITIONS=$(inherited) CMUX_CI_APP_HOST_ISOLATION_REQUIRED",
+                "build-for-testing",
+            ], arguments
+
+
 def run_app_host_unit_test_step(
     shard_mode: str = "selectors",
 ) -> tuple[subprocess.CompletedProcess[str], bool]: