diff --git a/.github/workflows/gatelock-tests.yml b/.github/workflows/gatelock-tests.yml
index f82779b..929270b 100644
--- a/.github/workflows/gatelock-tests.yml
+++ b/.github/workflows/gatelock-tests.yml
@@ -29,5 +29,13 @@ jobs:
       - name: Install dependencies
         run: pip install -r requirements.txt
 
+      # The viewport, fit-check and keyboard tests drive real Tk: focus
+      # delivery, canvas scrolling and widget heights are Tk's own behaviour,
+      # and mocking them is what let a viewport that scrolled itself pass
+      # every check for a week. Headless they skip, silently taking coverage
+      # with them -- so the runner gets a display instead.
+      - name: Install a virtual X display
+        run: sudo apt-get update && sudo apt-get install -y xvfb
+
       - name: Run tests with coverage
-        run: python -m pytest -v
+        run: xvfb-run -a -s "-screen 0 1600x1200x24" python -m pytest -v