I want to create a mainwindow with rounded corners. When I create a qwebenginview whose parent is the mainwindow, the rounded corners will be filled with black color

On my PC (windows 7):

Mainwindow with Qt::WA_TranslucentBackground atrribute,works exactally right under normal conditionI. The corners were transparent, app border was in correct shape.

But when I create a QWebengineview object whose parent is th mainwindow or other qwidget nherited from the mainwindow pointer, the rounded corners will be filled with black color,seems not be transparent.

the problem doesn't appear on windows 10. The transparent corner doesn't change to be black filled.

Is there any bug with qt on win7 OS？ I want the app display with round corner correctly on win7 macx win10.