fix(camera): let a two-finger tilt win over finger-spacing drift

Above 300 km the guard treated any shrinking pinch distance as an outward
zoom, so a two-finger tilt whose fingers drifted slightly closer had its
pitch overwritten. A two-finger gesture now counts as a zoom only when the
spacing changes more than the fingers' midpoint moves.

docs/CURRENT-STATE.md

@@ -54,8 +54,10 @@ Above 300 km, outward movement from the controller's configured zoom bindings
 down towards 8° below the Earth's limb. Large steps reach the limb immediately;
 the remaining margin eases into place, including after zooming stops or reaches
 the ceiling. Continuous outward inertia remains guarded without a time cutoff.
-Parallel two-finger tilts, modifier-specific tilts and other camera gestures
-cancel the correction. Explicit navigation handoffs, including toolbar tilt,
+Two-finger tilts, modifier-specific tilts and other camera gestures cancel the
+correction. A two-finger gesture counts as a pinch zoom only while the finger
+spacing changes more than the fingers move up or down, so spacing drift during
+a tilt is ignored. Explicit navigation handoffs, including toolbar tilt,
 cancel it before animation begins. Zooming in, disabled zoom/input, non-3D modes, tracked
 transforms and flights also cancel it; external pose changes cancel stationary
 settling. Oblique views below 300 km are left alone.

src/app/zoomOutGuard.js

@@ -47,10 +47,20 @@ function readInput(aggregator, bindings) {
   for (const { type, modifier } of INPUT_BINDINGS) {
     if (!aggregator.isMoving(type, modifier)) continue;
     const movement = aggregator.getMovement(type, modifier);
-    const distance =
-      type === Cesium.CameraEventType.PINCH ? movement.distance : movement;
+    const pinch = type === Cesium.CameraEventType.PINCH;
+    const distance = pinch ? movement.distance : movement;
     const delta = distance.endPosition.y - distance.startPosition.y;
-    if (matchesZoomBinding(bindings, type, modifier) && delta !== 0) {
+    // Cesium tilts by the fingers' midpoint while zooming by their spacing,
+    // both in the same quarter-pixel units. Fingers rarely keep a perfectly
+    // even spacing, so the larger of the two movements decides the gesture.
+    const tilt = pinch
+      ? movement.angleAndHeight.endPosition.y -
+        movement.angleAndHeight.startPosition.y
+      : 0;
+    if (
+      matchesZoomBinding(bindings, type, modifier) &&
+      Math.abs(delta) > Math.abs(tilt)
+    ) {
       outward ||= delta < 0;
       cancel ||= delta > 0;
     } else {

src/app/zoomOutGuard.test.mjs

@@ -245,6 +245,33 @@ test('parallel two-finger tilt is not mistaken for pinch zoom', () => {
   assert.equal(f.camera.views.length, 0);
 });
 
+test('two-finger tilt with finger-spacing drift is left to Cesium', () => {
+  for (const [spacing, tilt] of [
+    [-0.01, 3],
+    [-2, 3],
+    [-3, -3],
+  ]) {
+    const f = guardFixture();
+    f.frame(500_000);
+    f.movement(PINCH, spacing, { tiltDeltaY: tilt });
+    // Real Cesium's two-finger tilt climbs and picks its own pitch.
+    f.camera.pitch = Cesium.Math.toRadians(-26.07746);
+    f.frame(529_406);
+    assert.equal(f.camera.views.length, 0, `${spacing}, ${tilt}`);
+    f.dispose();
+  }
+});
+
+test('a pinch that closes the fingers more than it moves them still zooms out', () => {
+  // One finger still, the other sliding towards it: the midpoint moves half as far.
+  const f = guardFixture();
+  f.frame(500_000);
+  f.movement(PINCH, -10, { tiltDeltaY: -5 });
+  f.frame(700_000);
+  assert.equal(f.camera.views.length, 1);
+  f.dispose();
+});
+
 test('unconfigured wheel modifiers and one-finger drag do not enable the guard', () => {
   for (const [type, modifier] of [[WHEEL, SHIFT], [WHEEL, ALT], [LEFT_DRAG]]) {
     const f = guardFixture();