If I put QQuickWidget, some QWidget and vertical QSpacerItem in QVBoxLayout and set QQuickWidget::ResizeMode = SizeViewToRootObject, I expect that QQuickWidget will have a size of its content and QWidget will be placed just below it.

This is true unless content of QQuickWidget dynamically changes its size (for example some controls are shown/hidden). In this case, in spite QQuickWidget reports correct new sizeHint() the QVBoxLayout doesn't rearrange QWidget's position. It stays the same which could lead to empty spaces between widgets or widgets overlapping. It looks like QVBoxLayout doesn't understand that QQuickWidget has SizeViewToRootObject and thinks it has SizeRootObjectToView instead.

Sample project is attached.