Scalable UI

Overall goal:

Improve support for scalable UI by adding support for scaling at the QWindow level. This will enable fine-grained control over where and when scaling is applied, using C++ or QML API. In addition we can also support more advanced scaling modes where Qt computes the scale factor based on window size hints.

Existing support:

• Coordinate system scaling at the QScreen level
• API: setting QT_SCALE_FACTOR at application startup
• Graphics pipeline supports drawing at any DPR, with some limitations (widgets/raster, native styles)
• Qt for WebAssembly supports page zoom: can be used as a demo/test case.

Proposed changes:

• Formalize/document coordinate system scaling by introducing internal and external coordinate systems. The existing QScreen coordinate system scaling implementation uses this approach, although it is not documented using these terms.
• Add support for scaling the QWindow coordinate system.
• Add C++ API for setting per-window scale factors
• Add support for computing the window scale factor based on minimum/maximum size and available size.

Out of scope:

• Adding additional requirements for raster / widgets / styles beyond what is already required for e.g. Windows support.

Coordinate System Explainer:

 TODO

Prototype:

https://codereview.qt-project.org/c/qt/qtbase/+/440265