#include <QMdiArea>
#include <QMdiSubWindow>
#include <QStringListModel>
#include <QTableView>
#include <QLabel>
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QMdiArea mdiArea;
    mdiArea.show();

    QStringList stringList;
    for (int i = 1; i <= 10; i++)
        stringList << QString("bla").repeated(i);

    QTableView *tableView = new QTableView(&mdiArea);
    QStringListModel *model = new QStringListModel(stringList, tableView);
    tableView->setMinimumSize(800, 600);
    tableView->setModel(model);

    QMdiSubWindow *tableWindow = mdiArea.addSubWindow(tableView, Qt::Window);
    tableWindow->show();

    QMdiSubWindow *labelWindow = mdiArea.addSubWindow(new QLabel("<h1>oh my grid!<br>for what?</h1>"), Qt::Window);
    labelWindow->move(300, 100);
    labelWindow->show();

    return a.exec();
}

void QTableView::paintEvent(QPaintEvent *event)
{
    uint x = horizontalHeader->length() - horizontalHeader->offset() - (rightToLeft ? 0 : 1);
    uint y = verticalHeader->length() - verticalHeader->offset() - 1;
    ...
    for (QRect dirtyArea : region) {
        dirtyArea.setBottom(qMin(dirtyArea.bottom(), int(y)));
        if (rightToLeft) {
            dirtyArea.setLeft(qMax(dirtyArea.left(), d->viewport->width() - int(x)));
        } else {
            dirtyArea.setRight(qMin(dirtyArea.right(), int(x)));
        }
       // and dirtyArea may be invalid: 
       // dirtyArea.right() == x && x < dirtyArea.left()
       // dirtyArea.bottom() == y && y < dirtyArea.top()
        ...
        if (showGrid) {
            ...
            // even if dirtyArea.right() < dirtyArea.left()
            painter.drawLine(dirtyArea.left(), rowY + rowh, dirtyArea.right(), rowY + rowh);
            ...
            // even if dirtyArea.bottom() < dirtyArea.top()
            painter.drawLine(colp, dirtyArea.top(), colp, dirtyArea.bottom());
        }
    }
}