[REG 5.2.1->5.4.0] Bad rendering of combining diacritics

The combining breve (unicode 0306) is not anymore handled correctly. It should be placed ABOVE the character placed before it.
It was correctly handled with Qt5.2.1 and not with 5.4.0
I have found a previous report on the same type of regression, a few years ago between 4.7 and 4.8.
I have not tried with any other combining character, but I assume that if it affects the "combining breve" (which is crucial for the project Collatinus we are working on : http://outils.biblissima.fr/collatinus/) it will affects all of them.
Attached :

• a very simple code that displays 2 words in a QTextEdit
• the result with Qt 5.2 (correct)
• the result with Qt 5.4 (incorrect)