QHighDpiScaling Code Structure for Qt 6

QHighDpiScaling implements the scaling between the device independent and native coordinate systems.

Today, this is implemented in lower level QtGui / upper level of QPA (more details in option 0 below). The implementation has the following characteristics:

• It's possible to maintain a platform plugin without specific knowledge about QHighDpiScaling
• It's possible to maintain upper layers of Qt Gui (and above) without specific knowledge about QHighDpiScaling
• The implementation is cross-platform, and can be tested/simulated on any platform.

The code is isolated to a specific layer in the QtGui stack - at least to a certain degree. Is it possible to isolate it further?

Option 0: (the current implementation)

• Insert scaling calls when calling QPlatform* API
• Insert scaling calls when creating QWindowSystemInterface events
• Implement event handling code (related to QHighDpiScaling) in QGuiApplication

Example pseudo-code:

QRect QWindow::geometry() {
   return QHighDpiScaling::fromNative(
        platformWindow->geometry(), this);
}
void QWindow::setGeometry(QRect geometry) {
   platformWindow->setGeometry(
       QHighDpiScaling::toNative(geometry, this));
}

{code:java}
QT_DEFINE_QPA_EVENT_HANDLER(bool, handleMouseEvent, window, pos) {
    auto *event = new QWindowSystemInterfacePrivate::MouseEvent(
        window, QHighDpi::fromNative(pos, window));
}

Option 1: Move QHighDpiScaling calls down to the QPA level

• QPlatform* API returns geometry in device independent pixels - no scaling calls in QWindow.
• Insert scaling calls when creating QWindowSystemInterface events
• Implement event handling code (related to QHighDpiScaling) in the QWindowSystemInterface::handle* functions, instead of in QGuiApplication

This creates a conflict: QPlatformWindow::geometry() now needs to perform the scaling, but is at the same time overridden by the platforms. One way to resolve this is to add new "public" QPlatformWindow API:

QRect QPlatformWindow::deviceIndependentGeometry() {
    return QHighDpi::fromNative(this->geometry(), this);
}

QWindow then becomes:

QRect QWindow::geometry() {
    return platformWindow->deviceIndependentGeometry();
}

Option 2: Move QHighDpiScaling calls down to the QPA level and leave geometry() untouched

• QPlatform* API returns geometry in device independent pixels - no scaling calls in QWindow.
• Limits dealing with scaling to platforms that need it

QRect QWindowsWindow::geometry() {
    return QHighDpi::fromNative(m_nativeHandle->geometry(), this); // scaling needed 
}

QRect QCocoaWindow::geometry() {
    return m_nativeHandle->geometry(); // no scaling needed
}

QRect QWindow::geometry() {
    return platformWindow->geometry(); // no need to think about scaling, platform layer always reports device independent pixels, isolating the logic
}