Bug 1392361 - Fix zooming sensitivity on macOS

The original code would get a long sequence of miniscule "tick" values while
pinch-zooming, and each tick value would cause a 1.1x zoom. So even the smallest
pinch gesture on a trackpad would cause high amounts of zoom. This patch
accumulates the wheel deltas until they reach an integer threshold (with a
tweak of the scaling factor to make it feel more natural) at which point it
triggers the zoom based on the integer component of the accumulated delta. The
fractional part is retained in the accumulator.

Author: staktrace

web/app.js

@@ -26,7 +26,7 @@ import {
   MAX_SCALE,
   MIN_SCALE,
   noContextMenuHandler,
-  normalizeWheelEventDelta,
+  normalizeWheelEventDirection,
   parseQueryString,
   PresentationModeState,
   ProgressBar,
@@ -237,6 +237,7 @@ const PDFViewerApplication = {
   contentDispositionFilename: null,
   triggerDelayedFallback: null,
   _saveInProgress: false,
+  _wheelUnusedTicks: 0,
 
   // Called once when the document is loaded.
   async initialize(appConfig) {
@@ -1835,6 +1836,22 @@ const PDFViewerApplication = {
     _boundEvents.windowBeforePrint = null;
     _boundEvents.windowAfterPrint = null;
   },
+
+  accumulateWheelTicks(ticks) {
+    // If the scroll direction changed, reset the accumulated wheel ticks.
+    if (
+      (this._wheelUnusedTicks > 0 && ticks < 0) ||
+      (this._wheelUnusedTicks < 0 && ticks > 0)
+    ) {
+      this._wheelUnusedTicks = 0;
+    }
+    this._wheelUnusedTicks += ticks;
+    const wholeTicks =
+      Math.sign(this._wheelUnusedTicks) *
+      Math.floor(Math.abs(this._wheelUnusedTicks));
+    this._wheelUnusedTicks -= wholeTicks;
+    return wholeTicks;
+  },
 };
 
 let validateFileURL;
@@ -2492,13 +2509,34 @@ function webViewerWheel(evt) {
 
     const previousScale = pdfViewer.currentScale;
 
-    const delta = normalizeWheelEventDelta(evt);
+    const delta = normalizeWheelEventDirection(evt);
+    let ticks = 0;
+    if (
+      evt.deltaMode === WheelEvent.DOM_DELTA_LINE ||
+      evt.deltaMode === WheelEvent.DOM_DELTA_PAGE
+    ) {
+      // For line-based devices, use one tick per event, because different
+      // OSs have different defaults for the number lines. But we generally
+      // want one "clicky" roll of the wheel (which produces one event) to
+      // adjust the zoom by one step.
+      if (Math.abs(delta) >= 1) {
+        ticks = Math.sign(delta);
+      } else {
+        // If we're getting fractional lines (I can't think of a scenario
+        // this might actually happen), be safe and use the accumulator.
+        ticks = PDFViewerApplication.accumulateWheelTicks(delta);
+      }
+    } else {
+      // pixel-based devices
+      const PIXELS_PER_LINE_SCALE = 30;
+      ticks = PDFViewerApplication.accumulateWheelTicks(
+        delta / PIXELS_PER_LINE_SCALE
+      );
+    }
 
-    const MOUSE_WHEEL_DELTA_PER_PAGE_SCALE = 3.0;
-    const ticks = delta * MOUSE_WHEEL_DELTA_PER_PAGE_SCALE;
     if (ticks < 0) {
       PDFViewerApplication.zoomOut(-ticks);
-    } else {
+    } else if (ticks > 0) {
       PDFViewerApplication.zoomIn(ticks);
     }
 

web/ui_utils.js

@@ -636,13 +636,18 @@ function getPDFFileNameFromURL(url, defaultFilename = "document.pdf") {
   return suggestedFilename || defaultFilename;
 }
 
-function normalizeWheelEventDelta(evt) {
+function normalizeWheelEventDirection(evt) {
   let delta = Math.sqrt(evt.deltaX * evt.deltaX + evt.deltaY * evt.deltaY);
   const angle = Math.atan2(evt.deltaY, evt.deltaX);
   if (-0.25 * Math.PI < angle && angle < 0.75 * Math.PI) {
     // All that is left-up oriented has to change the sign.
     delta = -delta;
   }
+  return delta;
+}
+
+function normalizeWheelEventDelta(evt) {
+  let delta = normalizeWheelEventDirection(evt);
 
   const MOUSE_DOM_DELTA_PIXEL_MODE = 0;
   const MOUSE_DOM_DELTA_LINE_MODE = 1;
@@ -1017,6 +1022,7 @@ export {
   scrollIntoView,
   watchScroll,
   binarySearchFirstItem,
+  normalizeWheelEventDirection,
   normalizeWheelEventDelta,
   animationStarted,
   WaitOnType,