Qt align tops of widgets in separate layouts

alignAcrossLayouts.cpp

// in Widget A, residing in some other layout, but part of the same window as widget B
// someBox is a widget somewhere in widgetA
void WidgetA::resizeEvent(QResizeEvent *event)
{
	widgetB->setBoxYPos(someBox->mapTo(this->window(), someBox->rect().topLeft()).y());
	QWidget::resizeEvent(event);
}


// in widget B, which has a member QVBoxLayout* mainLayout
// someWidget is a widget somewhere in Widget B's mainLayout
void WidgetB::setBoxYPos(int y)
{
	mainLayout->setSpacing(mainLayout->spacing() + y - someWidget->mapTo(this->window(), someWidget->rect().topLeft()).y());
}

// note: use rect().topLeft() instead of pos()