Comment 2 for bug 1436074

The tab previews are implemented as image captures of the webviews, taken with the Item::grabToImage() method (http://doc.qt.io/qt-5/qml-qtquick-item.html#grabToImage-method). Here, the captures stored on disk are valid images (correct dimensions), but they are fully black, so it looks like something goes wrong when capturing the item or when saving the capture to disk.