Running Mojave in DarkMode I have noticed that the Slider is not correctly displayed if the window is resized in the Open event. The slider circle is shown but not the path / line.
To reproduce just place a slider in a window and input some new window height at the Open event.
It works fine if size is changed in Activate event or later but not at the Open event.
I always used Open to set the new window size as Activate may fire also later and I would need to control if it's first time.