Too large SVG icon on low-DPI monitor on Retina Mac with QIcon::fromTheme and AA_UseHighDpiPixmaps

Originally reported on the development mailing list here:

https://lists.qt-project.org/pipermail/development/2019-April/035599.html

The following test case:

#include <QApplication>
#include <QIcon>
#include <QToolButton>

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

    app.setAttribute(Qt::AA_UseHighDpiPixmaps);

    QIcon::setThemeName("mytheme");

    // Create icon from file name -> Size of pixmap is correct on both screens
    QToolButton button1;
    button1.setIcon(QIcon("icons/mytheme/scalable/actions/test-icon.svg"));
    button1.show();

    // Create icon from theme -> Size of pixmap is correct only on primary screen
    QToolButton button2;
    button2.setIcon(QIcon::fromTheme("test-icon"));
    button2.show();

    return app.exec();
}

Will produce this result when run on an external lower-DPI monitor hooked up to a Retina Mac:

Note how the icon created using QIcon("icons/mytheme/scalable/actions/test-icon.svg") (left) has the correct size, while the one created using QIcon::fromTheme("test-icon") (right) has the wrong size for that monitor (the size is correct for the internal HiDPI screen).

The app.setAttribute(Qt::AA_UseHighDpiPixmaps) is crucial. Without it, both icons have correct size on both monitors.

There seems to be some bug in the icon engine used when using an icon through a theme that makes it not respect the device pixel ratio of the current screen, but instead use that of the primary screen (the built-in HiDPI screen).

I've browsed through bug reports on QIcon + HiDPI + AA_UseHighDpiPixmaps but not found one quite like this. Sorry in advance if it's a duplicate!

I'm attaching the full test case as a ZIP.