Accessibility: character position in pixels doesn't match actual caret's Y coordinates

If you open any source file in Qt Creator's editor and move the caret,
the focus' position as reported through the At-SPI bus is incorrect:
whilst the horizontal coordinates are updated accurately, the vertical
(Y) coordinates remain stuck on the first line.

In the attached screenshot, the caret is on line 5 whereas the reported
focus position (highlighted in red) is still on line 1.

This is important from an accessibility point of view because assistive
technologies rely on that output to provide visually-impaired users with
focus tracking (e.g. moving zoom, screen readers etc.).

Also attached is a Python script that can help reproduce the problem.
It depends on Cairo, GTK 3 and py-atspi.