The Qt documentation states:

To ensure that the popup is positioned within the bounds of the enclosing window, the margins property can be set to a non-negative value.

However, this is not the case when y is set and the Popup is reopen.

This can be demonstrated with the Qt Quick Controls 2 Gallery example and adding items to the ComboBox and using the Material style.

The first time the Combo is opened, its popup stays within the bounds of the window (see 1st_time.png).
When reopening the combo, this is not the case anymore: notice how item "L" is not visible in 2nd_time.png.

This does not happen when:

• y is set to 0, or y is not set
• and transformOrigin: Item.Top is not set

Notably it does not happen for ComboBox when using the Universal style.

I was not able to reproduce with Qt 5.13.2 or Qt 5.12.8