From 2d434ae233f42e6aa2ccc3ce0819d243adb8e726 Mon Sep 17 00:00:00 2001
From: Estr <e@tausendsassa.io>
Date: Thu, 6 Feb 2025 18:13:01 +0100
Subject: [PATCH 1/3] Implement momentum panning

---
 src/zoom.js | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 62 insertions(+), 3 deletions(-)

diff --git a/src/zoom.js b/src/zoom.js
index d5643882..44c325e4 100644
--- a/src/zoom.js
+++ b/src/zoom.js
@@ -67,7 +67,8 @@ export default function() {
       touchDelay = 500,
       wheelDelay = 150,
       clickDistance2 = 0,
-      tapDistance = 10;
+      tapDistance = 10,
+      momentum = false;
 
   function zoom(selection) {
     selection
@@ -269,6 +270,17 @@ export default function() {
 
   function mousedowned(event, ...args) {
     if (touchending || !filter.apply(this, arguments)) return;
+
+    var isDown;
+    var startX, startY;
+    var velocityX = 0, velocityY = 0;
+    var velocityDecay = 0.75;
+    var velocityLimit = 0.5;
+    var momentumID;
+    var translation;
+
+    isDown = true;
+
     var currentTarget = event.currentTarget,
         g = gesture(this, args, true).event(event),
         v = select(event.view).on("mousemove.zoom", mousemoved, true).on("mouseup.zoom", mouseupped, true),
@@ -282,21 +294,64 @@ export default function() {
     interrupt(this);
     g.start();
 
+    startX = x0;
+    startY = y0;
+
+   cancelMomentumTracking();
+
+    function beginMomentumTracking() {
+      cancelMomentumTracking();
+      momentumID = requestAnimationFrame(momentumLoop);
+    }
+
+    function cancelMomentumTracking() {
+      cancelAnimationFrame(momentumID);
+    }
+
+    function momentumLoop() {
+      velocityX *= velocityDecay;
+      velocityY *= velocityDecay;
+
+      if (translation) {
+        translation.x += velocityX;
+        translation.y += velocityY;
+
+        g.event(event)
+          .zoom("mouse", constrain(translation, g.extent, translateExtent));
+      }
+
+      if (Math.abs(velocityX) > velocityLimit || Math.abs(velocityY) > velocityLimit) {
+        momentumID = requestAnimationFrame(momentumLoop);
+      }
+    }
+
     function mousemoved(event) {
       noevent(event);
       if (!g.moved) {
         var dx = event.clientX - x0, dy = event.clientY - y0;
         g.moved = dx * dx + dy * dy > clickDistance2;
       }
+
+      translation = translate(g.that.__zoom, g.mouse[0] = pointer(event, currentTarget), g.mouse[1]);
+
       g.event(event)
-       .zoom("mouse", constrain(translate(g.that.__zoom, g.mouse[0] = pointer(event, currentTarget), g.mouse[1]), g.extent, translateExtent));
+        .zoom("mouse", constrain(translation, g.extent, translateExtent));
+
+      if (momentum) {
+        if (!isDown) return;
+        velocityX = event.clientX - startX;
+        velocityY = event.clientY - startY;
+      }
     }
 
     function mouseupped(event) {
       v.on("mousemove.zoom mouseup.zoom", null);
+      isDown = false;
       dragEnable(event.view, g.moved);
       noevent(event);
       g.event(event).end();
+
+      momentum && beginMomentumTracking();
     }
   }
 
@@ -387,7 +442,7 @@ export default function() {
       if (g.taps === 2) {
         t = pointer(t, this);
         if (Math.hypot(touchfirst[0] - t[0], touchfirst[1] - t[1]) < tapDistance) {
-          var p = select(this).on("dblclick.zoom");
+          var p = select(this).on('dblclick.zoom');
           if (p) p.apply(this, arguments);
         }
       }
@@ -406,6 +461,10 @@ export default function() {
     return arguments.length ? (touchable = typeof _ === "function" ? _ : constant(!!_), zoom) : touchable;
   };
 
+  zoom.momentum = function(_) {
+    return arguments.length ? (momentum = _, zoom) : momentum;
+  };
+
   zoom.extent = function(_) {
     return arguments.length ? (extent = typeof _ === "function" ? _ : constant([[+_[0][0], +_[0][1]], [+_[1][0], +_[1][1]]]), zoom) : extent;
   };

From 65b0d2a493a326a1015921f4be95090b49ad20e3 Mon Sep 17 00:00:00 2001
From: Estr <e@tausendsassa.io>
Date: Thu, 6 Feb 2025 22:50:27 +0100
Subject: [PATCH 2/3] Implement momentum panning for touch events

---
 src/zoom.js | 137 +++++++++++++++++++++++++++++-----------------------
 1 file changed, 77 insertions(+), 60 deletions(-)

diff --git a/src/zoom.js b/src/zoom.js
index 44c325e4..35d925c0 100644
--- a/src/zoom.js
+++ b/src/zoom.js
@@ -11,7 +11,7 @@ import noevent, {nopropagation} from "./noevent.js";
 // Ignore right-click, since that should open the context menu.
 // except for pinch-to-zoom, which is sent as a wheel+ctrlKey event
 function defaultFilter(event) {
-  return (!event.ctrlKey || event.type === 'wheel') && !event.button;
+  return (!event.ctrlKey || event.type === "wheel") && !event.button;
 }
 
 function defaultExtent() {
@@ -70,6 +70,57 @@ export default function() {
       tapDistance = 10,
       momentum = false;
 
+  var isDown;
+
+  var startX,
+      startY;
+
+  var velocityX = 0,
+      velocityY = 0;
+
+  var velocityDecay = 0.75;
+  var velocityLimit = 0.5;
+
+  var momentumID;
+  var translation;
+
+  function beginMomentumTracking(g) {
+    cancelMomentumTracking();
+    momentumID = requestAnimationFrame(() => momentumLoop(g));
+  }
+
+  function cancelMomentumTracking() {
+    cancelAnimationFrame(momentumID);
+  }
+
+  function momentumLoop(g) {
+    velocityX *= velocityDecay;
+    velocityY *= velocityDecay;
+
+    if (translation) {
+      translation.x += velocityX;
+      translation.y += velocityY;
+
+      g.zoom("mouse", constrain(translation, g.extent, translateExtent));
+    }
+
+    if (Math.abs(velocityX) > velocityLimit || Math.abs(velocityY) > velocityLimit) {
+      momentumID = requestAnimationFrame(() => momentumLoop(g));
+    }
+  }
+
+  function setStart(event) {
+    isDown = true;
+    startX = event.clientX;
+    startY = event.clientY;
+  }
+
+  function setVelocity(event) {
+    if (!isDown) return;
+    velocityX = event.clientX - startX;
+    velocityY = event.clientY - startY;
+  }
+
   function zoom(selection) {
     selection
         .property("__zoom", defaultTransform)
@@ -271,16 +322,6 @@ export default function() {
   function mousedowned(event, ...args) {
     if (touchending || !filter.apply(this, arguments)) return;
 
-    var isDown;
-    var startX, startY;
-    var velocityX = 0, velocityY = 0;
-    var velocityDecay = 0.75;
-    var velocityLimit = 0.5;
-    var momentumID;
-    var translation;
-
-    isDown = true;
-
     var currentTarget = event.currentTarget,
         g = gesture(this, args, true).event(event),
         v = select(event.view).on("mousemove.zoom", mousemoved, true).on("mouseup.zoom", mouseupped, true),
@@ -294,36 +335,8 @@ export default function() {
     interrupt(this);
     g.start();
 
-    startX = x0;
-    startY = y0;
-
-   cancelMomentumTracking();
-
-    function beginMomentumTracking() {
-      cancelMomentumTracking();
-      momentumID = requestAnimationFrame(momentumLoop);
-    }
-
-    function cancelMomentumTracking() {
-      cancelAnimationFrame(momentumID);
-    }
-
-    function momentumLoop() {
-      velocityX *= velocityDecay;
-      velocityY *= velocityDecay;
-
-      if (translation) {
-        translation.x += velocityX;
-        translation.y += velocityY;
-
-        g.event(event)
-          .zoom("mouse", constrain(translation, g.extent, translateExtent));
-      }
-
-      if (Math.abs(velocityX) > velocityLimit || Math.abs(velocityY) > velocityLimit) {
-        momentumID = requestAnimationFrame(momentumLoop);
-      }
-    }
+    setStart(event);
+    cancelMomentumTracking();
 
     function mousemoved(event) {
       noevent(event);
@@ -333,25 +346,17 @@ export default function() {
       }
 
       translation = translate(g.that.__zoom, g.mouse[0] = pointer(event, currentTarget), g.mouse[1]);
-
-      g.event(event)
-        .zoom("mouse", constrain(translation, g.extent, translateExtent));
-
-      if (momentum) {
-        if (!isDown) return;
-        velocityX = event.clientX - startX;
-        velocityY = event.clientY - startY;
-      }
+      g.event(event).zoom("mouse", constrain(translation, g.extent, translateExtent));
+      momentum && setVelocity(event);
     }
 
     function mouseupped(event) {
       v.on("mousemove.zoom mouseup.zoom", null);
-      isDown = false;
       dragEnable(event.view, g.moved);
       noevent(event);
       g.event(event).end();
-
-      momentum && beginMomentumTracking();
+      isDown = false;
+      momentum && beginMomentumTracking(g);
     }
   }
 
@@ -385,8 +390,13 @@ export default function() {
 
     if (touchstarting) touchstarting = clearTimeout(touchstarting);
 
+    setStart(t);
+    cancelMomentumTracking();
+
     if (started) {
-      if (g.taps < 2) touchfirst = p[0], touchstarting = setTimeout(function() { touchstarting = null; }, touchDelay);
+      if (g.taps < 2) touchfirst = p[0], touchstarting = setTimeout(function() {
+        touchstarting = null;
+      }, touchDelay);
       interrupt(this);
       g.start();
     }
@@ -404,20 +414,22 @@ export default function() {
       if (g.touch0 && g.touch0[2] === t.identifier) g.touch0[0] = p;
       else if (g.touch1 && g.touch1[2] === t.identifier) g.touch1[0] = p;
     }
-    t = g.that.__zoom;
+
+    var z = g.that.__zoom;
     if (g.touch1) {
       var p0 = g.touch0[0], l0 = g.touch0[1],
           p1 = g.touch1[0], l1 = g.touch1[1],
           dp = (dp = p1[0] - p0[0]) * dp + (dp = p1[1] - p0[1]) * dp,
           dl = (dl = l1[0] - l0[0]) * dl + (dl = l1[1] - l0[1]) * dl;
-      t = scale(t, Math.sqrt(dp / dl));
+      z = scale(z, Math.sqrt(dp / dl));
       p = [(p0[0] + p1[0]) / 2, (p0[1] + p1[1]) / 2];
       l = [(l0[0] + l1[0]) / 2, (l0[1] + l1[1]) / 2];
-    }
-    else if (g.touch0) p = g.touch0[0], l = g.touch0[1];
+    } else if (g.touch0) p = g.touch0[0], l = g.touch0[1];
     else return;
 
-    g.zoom("touch", constrain(translate(t, p, l), g.extent, translateExtent));
+    translation = translate(z, p, l);
+    g.zoom("touch", constrain(translation, g.extent, translateExtent));
+    momentum && setVelocity(t);
   }
 
   function touchended(event, ...args) {
@@ -428,7 +440,9 @@ export default function() {
 
     nopropagation(event);
     if (touchending) clearTimeout(touchending);
-    touchending = setTimeout(function() { touchending = null; }, touchDelay);
+    touchending = setTimeout(function() {
+      touchending = null;
+    }, touchDelay);
     for (i = 0; i < n; ++i) {
       t = touches[i];
       if (g.touch0 && g.touch0[2] === t.identifier) delete g.touch0;
@@ -447,6 +461,9 @@ export default function() {
         }
       }
     }
+
+    isDown = false;
+    momentum && beginMomentumTracking(g);
   }
 
   zoom.wheelDelta = function(_) {

From b6853570f988a3babfc29fef0e264563052cddc0 Mon Sep 17 00:00:00 2001
From: Estr <e@tausendsassa.io>
Date: Fri, 14 Feb 2025 09:29:50 +0100
Subject: [PATCH 3/3] Calculate average mousemove velocity

---
 src/zoom.js | 267 +++++++++++++++++++++++++++++-----------------------
 1 file changed, 151 insertions(+), 116 deletions(-)

diff --git a/src/zoom.js b/src/zoom.js
index 35d925c0..73ef3c4e 100644
--- a/src/zoom.js
+++ b/src/zoom.js
@@ -1,12 +1,12 @@
-import {dispatch} from "d3-dispatch";
-import {dragDisable, dragEnable} from "d3-drag";
-import {interpolateZoom} from "d3-interpolate";
-import {select, pointer} from "d3-selection";
-import {interrupt} from "d3-transition";
+import { dispatch } from "d3-dispatch";
+import { dragDisable, dragEnable } from "d3-drag";
+import { interpolateZoom } from "d3-interpolate";
+import { select, pointer } from "d3-selection";
+import { interrupt } from "d3-transition";
 import constant from "./constant.js";
 import ZoomEvent from "./event.js";
-import {Transform, identity} from "./transform.js";
-import noevent, {nopropagation} from "./noevent.js";
+import { Transform, identity } from "./transform.js";
+import noevent, { nopropagation } from "./noevent.js";
 
 // Ignore right-click, since that should open the context menu.
 // except for pinch-to-zoom, which is sent as a wheel+ctrlKey event
@@ -41,9 +41,9 @@ function defaultTouchable() {
 
 function defaultConstrain(transform, extent, translateExtent) {
   var dx0 = transform.invertX(extent[0][0]) - translateExtent[0][0],
-      dx1 = transform.invertX(extent[1][0]) - translateExtent[1][0],
-      dy0 = transform.invertY(extent[0][1]) - translateExtent[0][1],
-      dy1 = transform.invertY(extent[1][1]) - translateExtent[1][1];
+    dx1 = transform.invertX(extent[1][0]) - translateExtent[1][0],
+    dy0 = transform.invertY(extent[0][1]) - translateExtent[0][1],
+    dy1 = transform.invertY(extent[1][1]) - translateExtent[1][1];
   return transform.translate(
     dx1 > dx0 ? (dx0 + dx1) / 2 : Math.min(0, dx0) || Math.max(0, dx1),
     dy1 > dy0 ? (dy0 + dy1) / 2 : Math.min(0, dy0) || Math.max(0, dy1)
@@ -52,34 +52,37 @@ function defaultConstrain(transform, extent, translateExtent) {
 
 export default function() {
   var filter = defaultFilter,
-      extent = defaultExtent,
-      constrain = defaultConstrain,
-      wheelDelta = defaultWheelDelta,
-      touchable = defaultTouchable,
-      scaleExtent = [0, Infinity],
-      translateExtent = [[-Infinity, -Infinity], [Infinity, Infinity]],
-      duration = 250,
-      interpolate = interpolateZoom,
-      listeners = dispatch("start", "zoom", "end"),
-      touchstarting,
-      touchfirst,
-      touchending,
-      touchDelay = 500,
-      wheelDelay = 150,
-      clickDistance2 = 0,
-      tapDistance = 10,
-      momentum = false;
-
-  var isDown;
-
-  var startX,
-      startY;
-
-  var velocityX = 0,
-      velocityY = 0;
-
-  var velocityDecay = 0.75;
-  var velocityLimit = 0.5;
+    extent = defaultExtent,
+    constrain = defaultConstrain,
+    wheelDelta = defaultWheelDelta,
+    touchable = defaultTouchable,
+    scaleExtent = [0, Infinity],
+    translateExtent = [[-Infinity, -Infinity], [Infinity, Infinity]],
+    duration = 250,
+    interpolate = interpolateZoom,
+    listeners = dispatch("start", "zoom", "end"),
+    touchstarting,
+    touchfirst,
+    touchending,
+    touchDelay = 500,
+    wheelDelay = 150,
+    clickDistance2 = 0,
+    tapDistance = 10,
+    momentumEnabled = false;
+
+  var momentum = {
+    isMoving: false,
+    x: 0,
+    y: 0,
+    prevX: 0,
+    prevY: 0,
+    time: 0,
+    velocityX: 0,
+    velocityY: 0,
+    velocityDecay: 0.7,
+    velocityLimit: 0.5,
+    interval: null,
+  }
 
   var momentumID;
   var translation;
@@ -94,44 +97,62 @@ export default function() {
   }
 
   function momentumLoop(g) {
-    velocityX *= velocityDecay;
-    velocityY *= velocityDecay;
+    var decay = momentum.velocityDecay;
+    var limit = momentum.velocityLimit;
+
+    momentum.velocityX *= decay;
+    momentum.velocityY *= decay;
 
     if (translation) {
-      translation.x += velocityX;
-      translation.y += velocityY;
+      translation.x += momentum.velocityX;
+      translation.y += momentum.velocityY;
 
       g.zoom("mouse", constrain(translation, g.extent, translateExtent));
     }
 
-    if (Math.abs(velocityX) > velocityLimit || Math.abs(velocityY) > velocityLimit) {
+    if (Math.abs(momentum.velocityX) > limit || Math.abs(momentum.velocityY) > limit) {
       momentumID = requestAnimationFrame(() => momentumLoop(g));
     }
   }
 
-  function setStart(event) {
-    isDown = true;
-    startX = event.clientX;
-    startY = event.clientY;
+  function onStart({ clientX, clientY }) {
+    momentum.isMoving = true;
+    momentum.prevX = clientX;
+    momentum.prevY = clientY;
+    momentum.x = clientX;
+    momentum.y = clientY;
+    momentum.time = performance.now();
+  }
+
+  function onMove({ clientX, clientY, movementX, movementY }) {
+    if (!momentum.isMoving) return;
+
+    momentum.x = clientX;
+    momentum.y = clientY;
+
+    var time = performance.now();
+    var deltaTime = time - momentum.time;
+
+    momentum.velocityX = movementX / deltaTime * 10;
+    momentum.velocityY = movementY / deltaTime * 10;
+    momentum.time = time;
   }
 
-  function setVelocity(event) {
-    if (!isDown) return;
-    velocityX = event.clientX - startX;
-    velocityY = event.clientY - startY;
+  function onEnd() {
+    clearInterval(momentum.interval);
   }
 
   function zoom(selection) {
     selection
-        .property("__zoom", defaultTransform)
-        .on("wheel.zoom", wheeled, {passive: false})
-        .on("mousedown.zoom", mousedowned)
-        .on("dblclick.zoom", dblclicked)
+      .property("__zoom", defaultTransform)
+      .on("wheel.zoom", wheeled, { passive: false })
+      .on("mousedown.zoom", mousedowned)
+      .on("dblclick.zoom", dblclicked)
       .filter(touchable)
-        .on("touchstart.zoom", touchstarted)
-        .on("touchmove.zoom", touchmoved)
-        .on("touchend.zoom touchcancel.zoom", touchended)
-        .style("-webkit-tap-highlight-color", "rgba(0,0,0,0)");
+      .on("touchstart.zoom", touchstarted)
+      .on("touchmove.zoom", touchmoved)
+      .on("touchend.zoom touchcancel.zoom", touchended)
+      .style("-webkit-tap-highlight-color", "rgba(0,0,0,0)");
   }
 
   zoom.transform = function(collection, transform, point, event) {
@@ -153,7 +174,7 @@ export default function() {
   zoom.scaleBy = function(selection, k, p, event) {
     zoom.scaleTo(selection, function() {
       var k0 = this.__zoom.k,
-          k1 = typeof k === "function" ? k.apply(this, arguments) : k;
+        k1 = typeof k === "function" ? k.apply(this, arguments) : k;
       return k0 * k1;
     }, p, event);
   };
@@ -161,10 +182,10 @@ export default function() {
   zoom.scaleTo = function(selection, k, p, event) {
     zoom.transform(selection, function() {
       var e = extent.apply(this, arguments),
-          t0 = this.__zoom,
-          p0 = p == null ? centroid(e) : typeof p === "function" ? p.apply(this, arguments) : p,
-          p1 = t0.invert(p0),
-          k1 = typeof k === "function" ? k.apply(this, arguments) : k;
+        t0 = this.__zoom,
+        p0 = p == null ? centroid(e) : typeof p === "function" ? p.apply(this, arguments) : p,
+        p1 = t0.invert(p0),
+        k1 = typeof k === "function" ? k.apply(this, arguments) : k;
       return constrain(translate(scale(t0, k1), p0, p1), e, translateExtent);
     }, p, event);
   };
@@ -181,8 +202,8 @@ export default function() {
   zoom.translateTo = function(selection, x, y, p, event) {
     zoom.transform(selection, function() {
       var e = extent.apply(this, arguments),
-          t = this.__zoom,
-          p0 = p == null ? centroid(e) : typeof p === "function" ? p.apply(this, arguments) : p;
+        t = this.__zoom,
+        p0 = p == null ? centroid(e) : typeof p === "function" ? p.apply(this, arguments) : p;
       return constrain(identity.translate(p0[0], p0[1]).scale(t.k).translate(
         typeof x === "function" ? -x.apply(this, arguments) : -x,
         typeof y === "function" ? -y.apply(this, arguments) : -y
@@ -206,24 +227,31 @@ export default function() {
 
   function schedule(transition, transform, point, event) {
     transition
-        .on("start.zoom", function() { gesture(this, arguments).event(event).start(); })
-        .on("interrupt.zoom end.zoom", function() { gesture(this, arguments).event(event).end(); })
-        .tween("zoom", function() {
-          var that = this,
-              args = arguments,
-              g = gesture(that, args).event(event),
-              e = extent.apply(that, args),
-              p = point == null ? centroid(e) : typeof point === "function" ? point.apply(that, args) : point,
-              w = Math.max(e[1][0] - e[0][0], e[1][1] - e[0][1]),
-              a = that.__zoom,
-              b = typeof transform === "function" ? transform.apply(that, args) : transform,
-              i = interpolate(a.invert(p).concat(w / a.k), b.invert(p).concat(w / b.k));
-          return function(t) {
-            if (t === 1) t = b; // Avoid rounding error on end.
-            else { var l = i(t), k = w / l[2]; t = new Transform(k, p[0] - l[0] * k, p[1] - l[1] * k); }
-            g.zoom(null, t);
-          };
-        });
+      .on("start.zoom", function() {
+        gesture(this, arguments).event(event).start();
+      })
+      .on("interrupt.zoom end.zoom", function() {
+        gesture(this, arguments).event(event).end();
+      })
+      .tween("zoom", function() {
+        var that = this,
+          args = arguments,
+          g = gesture(that, args).event(event),
+          e = extent.apply(that, args),
+          p = point == null ? centroid(e) : typeof point === "function" ? point.apply(that, args) : point,
+          w = Math.max(e[1][0] - e[0][0], e[1][1] - e[0][1]),
+          a = that.__zoom,
+          b = typeof transform === "function" ? transform.apply(that, args) : transform,
+          i = interpolate(a.invert(p).concat(w / a.k), b.invert(p).concat(w / b.k));
+        return function(t) {
+          if (t === 1) t = b; // Avoid rounding error on end.
+          else {
+            var l = i(t), k = w / l[2];
+            t = new Transform(k, p[0] - l[0] * k, p[1] - l[1] * k);
+          }
+          g.zoom(null, t);
+        };
+      });
   }
 
   function gesture(that, args, clean) {
@@ -286,9 +314,9 @@ export default function() {
   function wheeled(event, ...args) {
     if (!filter.apply(this, arguments)) return;
     var g = gesture(this, args).event(event),
-        t = this.__zoom,
-        k = Math.max(scaleExtent[0], Math.min(scaleExtent[1], t.k * Math.pow(2, wheelDelta.apply(this, arguments)))),
-        p = pointer(event);
+      t = this.__zoom,
+      k = Math.max(scaleExtent[0], Math.min(scaleExtent[1], t.k * Math.pow(2, wheelDelta.apply(this, arguments)))),
+      p = pointer(event);
 
     // If the mouse is in the same location as before, reuse it.
     // If there were recent wheel events, reset the wheel idle timeout.
@@ -323,11 +351,11 @@ export default function() {
     if (touchending || !filter.apply(this, arguments)) return;
 
     var currentTarget = event.currentTarget,
-        g = gesture(this, args, true).event(event),
-        v = select(event.view).on("mousemove.zoom", mousemoved, true).on("mouseup.zoom", mouseupped, true),
-        p = pointer(event, currentTarget),
-        x0 = event.clientX,
-        y0 = event.clientY;
+      g = gesture(this, args, true).event(event),
+      v = select(event.view).on("mousemove.zoom", mousemoved, true).on("mouseup.zoom", mouseupped, true),
+      p = pointer(event, currentTarget),
+      x0 = event.clientX,
+      y0 = event.clientY;
 
     dragDisable(event.view);
     nopropagation(event);
@@ -335,7 +363,7 @@ export default function() {
     interrupt(this);
     g.start();
 
-    setStart(event);
+    onStart(event);
     cancelMomentumTracking();
 
     function mousemoved(event) {
@@ -347,7 +375,7 @@ export default function() {
 
       translation = translate(g.that.__zoom, g.mouse[0] = pointer(event, currentTarget), g.mouse[1]);
       g.event(event).zoom("mouse", constrain(translation, g.extent, translateExtent));
-      momentum && setVelocity(event);
+      momentumEnabled && onMove(event);
     }
 
     function mouseupped(event) {
@@ -355,18 +383,22 @@ export default function() {
       dragEnable(event.view, g.moved);
       noevent(event);
       g.event(event).end();
-      isDown = false;
-      momentum && beginMomentumTracking(g);
+
+      if (momentumEnabled) {
+        momentum.isMoving = false;
+        beginMomentumTracking(g);
+        onEnd();
+      }
     }
   }
 
   function dblclicked(event, ...args) {
     if (!filter.apply(this, arguments)) return;
     var t0 = this.__zoom,
-        p0 = pointer(event.changedTouches ? event.changedTouches[0] : event, this),
-        p1 = t0.invert(p0),
-        k1 = t0.k * (event.shiftKey ? 0.5 : 2),
-        t1 = constrain(translate(scale(t0, k1), p0, p1), extent.apply(this, args), translateExtent);
+      p0 = pointer(event.changedTouches ? event.changedTouches[0] : event, this),
+      p1 = t0.invert(p0),
+      k1 = t0.k * (event.shiftKey ? 0.5 : 2),
+      t1 = constrain(translate(scale(t0, k1), p0, p1), extent.apply(this, args), translateExtent);
 
     noevent(event);
     if (duration > 0) select(this).transition().duration(duration).call(schedule, t1, p0, event);
@@ -376,9 +408,9 @@ export default function() {
   function touchstarted(event, ...args) {
     if (!filter.apply(this, arguments)) return;
     var touches = event.touches,
-        n = touches.length,
-        g = gesture(this, args, event.changedTouches.length === n).event(event),
-        started, i, t, p;
+      n = touches.length,
+      g = gesture(this, args, event.changedTouches.length === n).event(event),
+      started, i, t, p;
 
     nopropagation(event);
     for (i = 0; i < n; ++i) {
@@ -390,7 +422,7 @@ export default function() {
 
     if (touchstarting) touchstarting = clearTimeout(touchstarting);
 
-    setStart(t);
+    onStart(t);
     cancelMomentumTracking();
 
     if (started) {
@@ -405,8 +437,8 @@ export default function() {
   function touchmoved(event, ...args) {
     if (!this.__zooming) return;
     var g = gesture(this, args).event(event),
-        touches = event.changedTouches,
-        n = touches.length, i, t, p, l;
+      touches = event.changedTouches,
+      n = touches.length, i, t, p, l;
 
     noevent(event);
     for (i = 0; i < n; ++i) {
@@ -418,9 +450,9 @@ export default function() {
     var z = g.that.__zoom;
     if (g.touch1) {
       var p0 = g.touch0[0], l0 = g.touch0[1],
-          p1 = g.touch1[0], l1 = g.touch1[1],
-          dp = (dp = p1[0] - p0[0]) * dp + (dp = p1[1] - p0[1]) * dp,
-          dl = (dl = l1[0] - l0[0]) * dl + (dl = l1[1] - l0[1]) * dl;
+        p1 = g.touch1[0], l1 = g.touch1[1],
+        dp = (dp = p1[0] - p0[0]) * dp + (dp = p1[1] - p0[1]) * dp,
+        dl = (dl = l1[0] - l0[0]) * dl + (dl = l1[1] - l0[1]) * dl;
       z = scale(z, Math.sqrt(dp / dl));
       p = [(p0[0] + p1[0]) / 2, (p0[1] + p1[1]) / 2];
       l = [(l0[0] + l1[0]) / 2, (l0[1] + l1[1]) / 2];
@@ -429,14 +461,14 @@ export default function() {
 
     translation = translate(z, p, l);
     g.zoom("touch", constrain(translation, g.extent, translateExtent));
-    momentum && setVelocity(t);
+    momentumEnabled && onMove(t);
   }
 
   function touchended(event, ...args) {
     if (!this.__zooming) return;
     var g = gesture(this, args).event(event),
-        touches = event.changedTouches,
-        n = touches.length, i, t;
+      touches = event.changedTouches,
+      n = touches.length, i, t;
 
     nopropagation(event);
     if (touchending) clearTimeout(touchending);
@@ -456,14 +488,17 @@ export default function() {
       if (g.taps === 2) {
         t = pointer(t, this);
         if (Math.hypot(touchfirst[0] - t[0], touchfirst[1] - t[1]) < tapDistance) {
-          var p = select(this).on('dblclick.zoom');
+          var p = select(this).on("dblclick.zoom");
           if (p) p.apply(this, arguments);
         }
       }
     }
 
-    isDown = false;
-    momentum && beginMomentumTracking(g);
+    if (momentumEnabled) {
+      momentum.isMoving = false;
+      beginMomentumTracking(g);
+      onEnd();
+    }
   }
 
   zoom.wheelDelta = function(_) {
@@ -479,7 +514,7 @@ export default function() {
   };
 
   zoom.momentum = function(_) {
-    return arguments.length ? (momentum = _, zoom) : momentum;
+    return arguments.length ? (momentumEnabled = _, zoom) : momentumEnabled;
   };
 
   zoom.extent = function(_) {