Otherwise the extension just disapear with some styles (see Issue #3795)
const char TOOLBAR_CSS[] =
"QToolBar { background: %1; margin: 0; padding: 0; border: none; border-bottom: 1px solid %2; spacing: 0; } "
"QToolBar QToolButton { background: %1; border: none; border-bottom: 1px solid %2; margin: 0; padding: 5px; } "
+ "QToolBar QToolBarExtension { padding:0; } "
"QToolBar QToolButton:checked { background: %3; color: %4; }";
static const float buttonSizeRatio = 1.618; // golden ratio