Stalls when displaying widget with QML contents

This problem was already solved when I joined the project at hand, but it is a rather typical scenario, so we will have a look at it here.

Context

In one Qt 5 project, we had a rather complicated GUI displaying an image in several different processing stages. As is often the case in image-processing applications, the user could define a region of interest (ROI) – typically a circle or a rectangle – to restrict an image operation to some part of the image. The ROI was displayed over the image and could be interactively moved and resized.

The ROI was implemented in QML because of the ease with which moving and resizing could be programmed. It was embedded into the...