Set more generous minimums. Flicker seems to have nothing to do with minimum size. I get it even for fairly large dimensions after scaling down from a much larger size. Similar to issues with zoom from about a month ago, before unc0rr fixed them.

#include <QApplication>

#include "mainform.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    MyWindow *mainWin = new MyWindow;
    mainWin->show();
    return app.exec();
}