The QtQuick based color dialog sometimes has a broken layout when shown with alpha channel slider

To reproduce:

• Start the systemdialogs example.
• Click tab "Color Dialog"
• Click "open"
• Press Escape
• Check "Show alpha channel"
• Click "open" again

Result is what you see in the attached screenshot.
The layout of the alpha channel slider is broken, the checkered background for the alpha channel is rotated 90 degrees, and its placed at the wrong position.