Custom Titlebar, Top-left, right border rounded; But left over part still visible

[rawfool]

I've implemented a custom titlebar using QToolBar. I've applied the style to it as shown below.

this->setStyleSheet("background: #646464; border: 0px; border-top-left-radius: 10px; border-top-right-radius: 10px;");

this->setStyleSheet("background: #646464; border: 0px; border-top-left-radius: 10px; border-top-right-radius: 10px;");

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Now I got the rounded edges, but the left-over part after the border-radius is still visible. How do I "cut" it or How do I make it transparent ?
Thank you.