Show QWidget as focused

Question

I've got four QLineEdit placed inside of a QLineEdits, where I want the first the parent to look as if it is in focus when any of the containing ones is selected. Note: I don't want the focus to actually change, just the "focus frame" (the thin blue border) to appear on the parent LineEdit.

I've tried to draw a rect, but while it works on Windows I'm running into issues of the drawn rectangle not looking like a proper rectangle on ex. Linux, where it is supposed to be rounded. Is there a way to fix this OR, if possible, just make it draw itself as focused despite focus not being on it?

Here's my attempt at drawing a custom rect, but haven't been able to make it successfully mirror the OS style properly.

            if (childHasFocus) {
            QPainter painter(this);

            QLineEdit textBox;
            QColor color = textBox.palette().color(QPalette::Highlight);

            painter.setPen(color);
            QRect rect;
            rect.setTopLeft(QPoint(0,0));
            rect.setWidth(this->width() - 1);
            rect.setHeight(this->height() - 1);
            painter.drawRect(rect);
        }

EDIT: Added an image of the desired look. Note that I'm trying to get it to look like other LineEdits focusframe independent of OS, so hardcoding a blue rectangle won't work due to ex. Linux having a rounded focusframe.

Desired look:

Answer 1

Here's how to do it. Its a very basic class that draws the focus frame if any of the childs have focus. On focus change, we do an update (which can probably be optimized a bit to avoid unnecessary repaints).

Screenshot:

class IPEdit : public QWidget
{
public:
    IPEdit(QWidget *parent = nullptr)
        : QWidget(parent)
    {
        delete layout();
        auto l = new QHBoxLayout(this);
        setFocusProxy(&a);
        setAttribute(Qt::WA_Hover);
        for (auto *w : {&a, &b, &c, &d}) {
            l->addWidget(w);
            w->installEventFilter(this);
        }
    }

    bool eventFilter(QObject *o, QEvent *e) override
    {
        if (e->type() == QEvent::FocusIn || e->type() == QEvent::FocusOut) {
            update();
        }
        return QWidget::eventFilter(o, e);
    }

    void paintEvent(QPaintEvent *e) override
    {
        QStyleOptionFrame opt;
        opt.initFrom(this);
        opt.frameShape = QFrame::StyledPanel;
        opt.state |= QStyle::State_Sunken;

        // clear mouseOver and focus state
        // update from relevant widgets
        opt.state &= ~(QStyle::State_HasFocus | QStyle::State_MouseOver);
        const auto widgets = {&a, &b, &c, &d};
        for (const QWidget *w : widgets) {
            if (w->hasFocus()) {
                opt.state |= QStyle::State_HasFocus;
            }
        }

        opt.rect = contentsRect();
        QPainter paint(this);
        paint.setClipRegion(e->region());
        paint.setRenderHints(QPainter::Antialiasing);
        style()->drawControl(QStyle::CE_ShapedFrame, &opt, &paint, this);
    }
private:
    QLineEdit a;
    QLineEdit b;
    QLineEdit c;
    QLineEdit d;
};