enum DisplayScale

The output scale a window is drawn at: framebuffer pixels per layout pixel.

A compositor drawing a window on a 1.5 screen renders it at 1.5 times its layout size and says so with a Scale event (InputEventKind.scale). Layout never changes — everything here stays in layout pixels — and most of a frame needs nothing from the app: shapes are drawn at device resolution by the renderer, and text is rasterised there from the same face at the device size. What the renderer cannot do is ask for an image at a size it was never decoded at, because the client picks the decode size. That is what this is for.

Two readings, for two kinds of caller:

• DrawList.displayScale is per window, and is what the framework's own Image(path:width:height:) uses at emit time — so a window on each of two screens decodes for its own.
• DisplayScale.main is the app's first window, for an app that sizes its own decodes in body (a dock's icons, a thumbnail cache). A change invalidates body, so a view that reads it is rebuilt with the new value. Apps with one window, which is nearly all of them, lose nothing.

Windowed apps have no source for it yet and stay at 1.