struct CanvasGesture

Which button started this: 0 left, 1 right, 2 middle, matching GLFW and the order the compositor forwards them in.

A canvas that only wants the primary button has to check, because every button arrives here — which is the honest default for a surface whose whole point is that the app decides what a press means. Middle-click paste and a right-click menu are the two that need it.

How many clicks this press continues: 1 for a single, 2 for a double, 3 for a triple. Carried through .moved and .ended like mods, so a double-click-and-drag knows all the way through that it is extending by words.

Here rather than left to the caller because the caller would have to find ClickCounter to get it, and one that reached for a timer of its own would disagree with the text editor and the title bar about what a double-click is — on the same desktop, under the same finger.

Current absolute canvas geometry, identical to the frame passed to paint. This makes local coordinates reversible without app-owned frame caches (for example, committing a drag range to chart values).

Relative to the canvas's own top-left, matching CanvasFrame. Can go negative, or past w/h, once a drag leaves the box.

Modifiers held at .began (see KeyMods). Not re-read for .moved/.ended — a drag can't ask again mid-flight, only a fresh press reflects current state, so this stays whatever .began saw.