This is strange in qt 5.15.0.  When I update my qt 5.14.2 project to qt 5.15.0,

all button widgets stop response. When I move back to qt 5.14.2, all become normal.

Finally, I found out that if I remove the style sheet setting, the program work again.

So I make a basic project that only has a button widget and a label widget. If I press the button, 

it will display a message in the label box. If I add style to the project from qt creator, the button

stop response. This error does not appear in window system.