Qt designer: How to add widget to a layout in the designer when the layout appears infinitely thin

Create QVBoxLayout, then add two QHBoxLayouts to it. In top QHBoxLayout add labels, in bottom one add stretch, button, stretch.

#include <QString>
#include <QApplication>
#include <QWidget>
#include <QPushButton>
#include <QVBoxLayout>
#include <QHBoxLayout>
#include <QLabel>
#include <QLocale>

int main(int argc, char** argv){
    QApplication app(argc, argv);

    QWidget widget;

    QVBoxLayout* vLayout = new QVBoxLayout(&widget);
    QHBoxLayout* topLayout = new QHBoxLayout();
    QHBoxLayout* bottomLayout = new QHBoxLayout();
    QLabel* label1 = new QLabel(QObject::tr("Label1"));
    QLabel* label2 = new QLabel(QObject::tr("Label2"));
    label1->setAlignment(Qt::AlignCenter);
    label2->setAlignment(Qt::AlignCenter);
    QPushButton* btn1 = new QPushButton(QObject::tr("The Button!!!!"));
    topLayout->addWidget(label1);
    topLayout->addWidget(label2);
    bottomLayout->addStretch();
    bottomLayout->addWidget(btn1);
    bottomLayout->addStretch();
    vLayout->addLayout(topLayout);
    vLayout->addLayout(bottomLayout);

    widget.show();

    return app.exec();
}

You can use the Object Inspector window, but you don't seem to be able to paste into a QFrame that has a locked layout.

So given your layout:

QMainWindow
- QWidget
  - QFrame (A)
    - QFrame (B)
    - QFrame (C)
    - QFrame (D)

Select QFrame D in the Object Inspector window and Cut it, right click QWidget and Break the layout (ctrl-0), then paste. Then re-apply the layout.