How to update size of nested QWidget?

[Maryu]

I'm using Qt 5.15 with C++17. I have a hierarchy of Widgets like hinted in the title:

MainWindow : QMainWindow
->centralWidget() = QScrollArea
   ->widget() = QWidget
      ->layout() = QVBoxLayout
         ->children() = [
            InnerWidget,
            InnerWidget,
            InnerWidget,
            ...
            ]

MainWindow : QMainWindow
->centralWidget() = QScrollArea
   ->widget() = QWidget
      ->layout() = QVBoxLayout
         ->children() = [
            InnerWidget,
            InnerWidget,
            InnerWidget,
            ...
            ]

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with

InnerWidget.children() = [ QLabel, QLabel ]

InnerWidget.children() = [ QLabel, QLabel ]

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.
I have set

InnerWidget::setSizePolicy(QSizePolicy::Minimum, QSizePolicy::Minimum)

InnerWidget::setSizePolicy(QSizePolicy::Minimum, QSizePolicy::Minimum)

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inside InnerWidget's constructor.

The QLabels inside InnerWidget are initialized with some QPixmap like

label->setPixmap(img);
label->setFixedSize(img.size());

label->setPixmap(img);
label->setFixedSize(img.size());

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_________________


This all works fine, however now I added an option to zoom all the pixmaps to the MainWindow's menubar. I connect to a signal MainWindow::zoom_inner(.., which itself calls InnerWidget::zoom(.. on every InnerWidget, which resizes the images / labels like this:

auto scaled = img.scaled(img.size() * factor);
label->setPixmap(scaled);
label->setFixedSize(scaled.size());

auto scaled = img.scaled(img.size() * factor);
label->setPixmap(scaled);
label->setFixedSize(scaled.size());

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Finally I update the size of the MainWindow by finding the maximum InnerWidget.frameGeometry().width() (= inner_max_width) and calling

void MainWindow::zoom_inner() {
  // ...
  setMaximumWidth(inner_max_width + centralWidget()->verticalScrollBar()->width() + 1);
  resize(maximumWidth(), size().height());
}

void MainWindow::zoom_inner() {
  // ...
  setMaximumWidth(inner_max_width + centralWidget()->verticalScrollBar()->width() + 1);
  resize(maximumWidth(), size().height());
}

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_________________

Now after calling MainWindow::zoom(.., the window seems to resize to the correct size, as do the QLabel's inside the InnerWidget's. However, the InnerWidget's themselves do not resize at all, they just stay the same size as before and I have no idea why. I tried calling many combinations of

adjustSize();
update();
layout()->update();
updateGeometry();

adjustSize();
update();
layout()->update();
updateGeometry();

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on any of the MainWindow,QScrollArea,InnerWidget but nothing happens.. even calling

InnerWidget::resize(new_size)

InnerWidget::resize(new_size)

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has no effect. Clearly InnerWidget::sizeHint() gives the correct value, because the MainWindow size after MainWindow::zoom(.. is correct. So why do the InnerWidget's refuse to resize to fit the QLabel's, even though sizePolicy is set to QSizePolicy::Minimum?

This seems like some sort of misunderstanding on my part so I'm hoping the answer is simple and doesn't need a MWE. I've tried the docs but couldn't find a solution.

[d_stranz]

sizePolicy is set to QSizePolicy::Minimum

This is telling the VBox layout that the widget's sizes are fixed at their minimum sizes. You need a size policy that includes "Expanding" or use "Preferred".