Patch https://codereview.qt-project.org/#/c/190803 (first released in Qt 5.9) introduced a regression in QtWebKit, which results in web fonts becoming invisible in QtWebKit on some pages (see attached screenshots). Web font is implemented as QRawFont object loaded from QByteArray. It was found, that replacing

fontData.toRawCFData()

with

fontData.toCFData()

in method

QCoreTextFontEngine::create(const QByteArray &, qreal, QFont::HintingPreference)

makes this bug disappear, which suggests there is an issue with life time management of the font data inside QtGui (QtWebKit operates on QRawFont public API and doesn't try to mess with fontengine internals)

It is not yet determined how to reproduce this bug outside of QtWebKit