To reproduce this, use the examples\graphicsview\diagramscene example and add the following code:-

//! [25]
void MainWindow::createToolbars()
{
//! [25]
/* Adding several extra toolbars enlarges the the toolbar area width.
Moving the toolbar fails, if the available mainwindow width is smaller
than the toolbar area width. */
for (int i = 0; i < 15; i++)
{
QString toolBarName = QString("My Toolbar %1").arg(i + 1);
QToolBar* myToolBar = addToolBar(toolBarName);
myToolBar->addWidget(new QLabel(toolBarName + QString(" takes some of space")));
}

Drag one of the additional toolbars to a lower position so it expands.The main window will contract. If you then try and drag it to the original location, the main window doesn't expand and the tool bar can only be floated.