Viewer trackpad gestures: two-finger scroll pans, pinch zooms

The Composition and Layer viewers took every scroll as a zoom, so a trackpad
could not pan. A trackpad's two-finger scroll now pans; a pinch,
Ctrl/Cmd+scroll, a mouse wheel and Alt + a trackpad scroll zoom around the
pointer.

Where the platform names the device (egui's MouseWheelSource, macOS), that
decides. Elsewhere browsers and some high-resolution wheels report a mouse
wheel in points, as a trackpad does, so the unit alone doesn't decide. A
scroll in points pans only once it has shown it is a trackpad: a touch phase,
movement on both axes at once, or a pinch. That holds until a wheel scrolls
in lines. Until then the vertical part zooms and the horizontal part pans.

ui.scroll gains unit (point|line), source (wheel|trackpad) and pinch so
agents can drive both gestures.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/control-protocol.md

@@ -103,7 +103,7 @@ Points are logical window coordinates. A target is `{id}` (the element's centre;
 | `ui.click` | `{id \| x,y, button?: left\|right\|middle, count?, modifiers?}` |
 | `ui.move` | `{id \| x,y}` |
 | `ui.drag` | `{from: target, to: target, steps? (12), modifiers?}`: press, move, release |
-| `ui.scroll` | `{id \| x,y, dx, dy}` |
+| `ui.scroll` | `{id \| x,y, dx?, dy?, unit?: point\|line, source?: wheel\|trackpad, pinch?, modifiers?}`: `line` is a mouse wheel (zooms the viewers); `point` (default) is a pixel-precise scroll; `source: trackpad` pans the viewers (as on macOS); with no `source` the viewers take a scroll in points as a wheel until one moves on both axes (a trackpad: from then on it pans); `pinch` is a trackpad pinch's zoom factor |
 | `ui.key` | `{key}`, for example `Space`, `F9`, `Cmd+Z`, `Cmd+Shift+Z`, `PageDown`, `Escape` |
 | `ui.type` | `{text}`: text input into the focused field |
 

docs/hardware-parity.md

@@ -23,7 +23,7 @@ monitors are missing, and two GPU start-up failures are open.
 | HDR display | HDR monitoring on EDR / HDR displays | tone-mapped SDR view; no EDR output | no HDR output | no | no | Colour engine handles HDR maths (Rec. 2100 PQ/HLG output files) |
 | Pen tablets (pressure, tilt) | yes, for Paint and Roto | Brushes panel has pressure options, but no tablet pressure is read | same | same | same | Paint strokes store pressure per vertex; input never fills it |
 | Multiple monitors | panels and windows across screens | panels move into their own native windows on any screen (Move Panel to New Window; desktop builds) | same | same (#486) | — | gap 20 |
-| Touch / trackpad gestures | pinch zoom in viewers | egui pointer and scroll; no pinch-to-zoom audited | — | — | — | — |
+| Touch / trackpad gestures | pinch zoom in viewers | Composition and Layer viewers: two-finger scroll pans, pinch zooms around the pointer (Alt + scroll and the mouse wheel zoom too); Timeline pinch zooms time | two-finger scroll pans once it moves diagonally (a pixel-precise wheel keeps zooming); pinch arrives as Ctrl + scroll and zooms | same | same | No rotate or swipe gestures; tested in `ui_viewer_trackpad.rs` (UI logic, not on hardware) |
 | Audio hardware | Core Audio / ASIO / WASAPI, device choice, latency | cpal output; preview sound late after a start offset (#504 open; #554 closed) | cpal | cpal (ALSA) | Web Audio | No input or device latency settings |
 | Machine learning acceleration | Apple Neural Engine / GPU for Object Matte, Roto Brush 3 | CPU inference (pure Rust, rayon) | CPU | CPU | CPU | No GPU inference for MobileSAM or MediaPipe |
 | Memory management | RAM reserve, disk cache | RAM Reserved, Reduce Cache When Low on Memory, disk cache | same | same | OPFS disk cache | good |