The size of window will increase when a QLabel use setPixmap() in QTabWidget

herocon

I use Qt5.15.2 in win11 and ubuntu20.04.
The Widget uses a layout, and the tabWidget also uses a layout. When the label uses the setPixmap(pix) function, and the size of pix is equal with the size of the label,the Widget will become bigger. When the width of pix's size is less than the width of the label's size minus 6, and the height of pix's size is less than the height of the label's size minus 6, everything works normally.

widget.h

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

QT_BEGIN_NAMESPACE
namespace Ui {
class Widget;
}
QT_END_NAMESPACE

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = nullptr);
    ~Widget();

protected:
    void timerEvent(QTimerEvent *e);
    void resizeEvent(QResizeEvent *e);

private:
    Ui::Widget *ui;
};
#endif // WIDGET_H

widget.cpp

#include "widget.h"
#include "ui_widget.h"
#include <QDebug>

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
    startTimer(1000);
    qDebug() << this->geometry();
}

Widget::~Widget()
{
    delete ui;
}

void Widget::timerEvent(QTimerEvent *e)
{
    qDebug() << ui->label->geometry();
    QPixmap p("/home/li/Pictures/1.jpg");
    ui->label->setPixmap(p.scaled(ui->label->size()));
}

void Widget::resizeEvent(QResizeEvent *e)
{
    qDebug() << "resizeEvent" <<ui->label->geometry();
}

herocon

If QStackedWidget is used in place of QTabWidget, this bug does not occur.

JonB

@herocon
Various widgets have things like content margins. Have you looked to see whether that is the difference?

herocon

@JonB said in The size of window will increase when a QLabel use setPixmap() in QTabWidget:

@herocon
Various widgets have things like content margins. Have you looked to see whether that is the difference?

QTabWidget is implemented based on QStackedWidget, but QStackedWidget does not have this bug. I suspect that the bug is caused by QTabWidget, or it could be that setPixmap() triggers a resize() of the QLayout.
The default content margins of QWidget is (0,0,0,0). The default content margins of QGridLayout is (11,11,11,11), but which shown in QtDesigner is (9,9,9,9).It may be another bug.Though there are various properties, the properties should not change the widget's size.

Pl45m4

@herocon

Your issue as you describe it sounds odd at first, but after looking at your code, I think I know what's going on:

If your idea of a bug originates from these two (or three) outputs:

[ 1 ]

Widget::Widget(QWidget *parent)
   : QWidget(parent)
   , ui(new Ui::Widget)
{
   qDebug() << this->geometry();
}

[ 2 ]

void Widget::timerEvent(QTimerEvent *e)
{
   qDebug() << ui->label->geometry();
}

[ 3 ]

void Widget::resizeEvent(QResizeEvent *e)
{
    qDebug() << "resizeEvent" << ui->label->geometry();
}

then there is no bug.

You cannot compare the geometry you retrieve within the widget's c'tor with the one after the widget is shown and layouts and other settings apply.

It might be different for a child widget with no content (QStackedWidget with no "pages"), since it doesn't need to expand that much... but for your Widget:
It keeps its inital size until it's shown and the resize happens (layouts apply... label child expands with pixmap)...
So the output you get from [ 1 ] will always be different from [ 2 ] and [ 3 ].

Implement the Widget::showEvent handler and put your "first test" [ 1 ] there. Then compare again.

TLDR:
Don't trust anything (screen) size related that you call from some widget's constructor. It will never be the size of the widget you see on your screen.

herocon

@Pl45m4 Sorry,I don't show the actual result of my code.The problem is not the information from debug().Actually, the application is getting higher every second in screen.It's not true.It changes the size of application which is parent of the layout.When I use the QStackedWidget and the stackedWidget alse has two pages and a label and layout, application remains unchanged, no matter how many times the timerEvent() occurs.
startTimer(100); And the result: alt text

Pl45m4

@herocon

Ok next attempt:

ui->label->setPixmap(p.scaled(ui->label->size()));

Each iteration scales the pixmap to current label size and then sets the pixmap to the label.
By doing this, the size of the label increases with the pixmap (because of border/frame... IIRC 1px per side by default, so 2px in total).
Then you take the new label size again in the next iteration and apply this new (by 2px) increased size to the pixmap... the label gets the pixmap, grows by about 2px in height and there you have your loop where your label and consequently the whole widget grows ;-)

So still no bug :)
And it's exactly the behavior you would expect from the debug output.
(The height increases by 2 each timer interval).
When using another layout setup, the layout/other widgets might "buffer" that, so the main app window does not grow.
But even there, the label itself should definitely grow when you periodically set content which has the exact same size as the outter boundings of the label.
Why though?!

Try (not tested):

int w = ui->label->width();
int h = ui->label->height();
ui->label->setPixmap(p.scaled(w - 2, h - 2));
// or this and set the pixmap only once
ui->label->setScaledContents(true);

Because a label with w=500, h=500 for example, can't display a pixmap with w=500, h=500. If you set a pixmap with the same size as the label itself, it will start to grow.