Create a frameless dialog by setting the Qt::FramelessWindowHint to true. Add three line edit based widgets (QSpinBox, QLineEdit, QTextEdit) to it. Override the mouse move event to move the window around the screen. Observe the blocky artefact to the left of the blinking cursor in the widget that has the focus. If the window is translucent you can see the background colour of screen under the cursor. Else, we see the background colour of the parent.

I have attached a sample program to demonstrate the problem. You can move the window around by holding the mouse button down on the grey border and moving it around. As you move the window, the background block artefact appears to the left of the blinking cursor. Populating the widgets makes the problem more obvious as the character to the left of the cursor disappears and appears as the window moves.