The attached example enforces a winId() on a QPlainTextEdit within a QMdiArea and activates scaling via env vars for 5.5/5.6.
The screen shots show that sth is messed up; probably related to the backing store (can't put my finger on it)?
Note: Screen shots were taken on low res screens , this is why the font sizes are different between 5.5/5.6 - this is not relevant to this report.
This also has been observed for QToolBox.
Discovered while working on https://codereview.qt-project.org/#/c/146248/