The demo apps used Java 21 + TestFX + JUnit5 + AssertJ + Monocle. Full write-up with configs and explanations here: dzone.com/articles/top...

Mistake 7: mixing domain logic with UI in tests.
Business logic should live in view models and be tested with plain JUnit. Keep UI tests for rendering and interaction.

Mistake 6: misconfigured headless CI.
Simply adding Monocle isn’t enough. Run with --add-opens, disable parallel UI tests, install native libs, and don’t assert fullscreen in headless.

Mistake 5: pixel-perfect equality across platforms.
HiDPI, AA, GPU vs Monocle in CI → colors shift. Add tolerance and never assume an exact RGB match across OS/CI.

Mistake 4: racing the FX event queue.
Assertions run before layout or scene switches complete.
❌ immediate assert
✅ wait for queue drain

Mistake 3: triggering handlers directly instead of simulating input. button. fire() bypasses pointer/focus bugs that real users hit.

Mistake 2: bootstrapping with mismatched loaders in Spring + TestFX.
TestFX owns the Stage, Spring owns the beans. If you let FXMLLoader use the wrong loader, you’ll get NPEs.

Mistake 1: touching UI off the FX thread.
JUnit runs on its own thread, but only the FX thread can mutate controls.