Adressed review.

Reusing transform
Changing either size of the canvas or clearing it
adding unit of hitTolerance to jsdoc

externs/olx.js

@@ -323,7 +323,7 @@ olx.AtPixelOptions.prototype.layerFilter;
 
 
 /**
- * Hit-detection tolerance. Pixels inside the radius around the given position
+ * Hit-detection tolerance in pixels. Pixels inside the radius around the given position
  * will be checked for features. This only works for the canvas renderer and
  * not for WebGL.
  * @type {number|undefined}

src/ol/interaction/select.js

@@ -168,7 +168,7 @@ ol.interaction.Select.prototype.getFeatures = function() {
 
 /**
  * Returns the Hit-detection tolerance.
- * @returns {number} Hit tolerance.
+ * @returns {number} Hit tolerance in pixels.
  * @api
  */
 ol.interaction.Select.prototype.getHitTolerance = function() {
@@ -291,7 +291,7 @@ ol.interaction.Select.handleEvent = function(mapBrowserEvent) {
  * Hit-detection tolerance. Pixels inside the radius around the given position
  * will be checked for features. This only works for the canvas renderer and
  * not for WebGL.
- * @param {number} hitTolerance Hit tolerance.
+ * @param {number} hitTolerance Hit tolerance in pixels.
  * @api
  */
 ol.interaction.Select.prototype.setHitTolerance = function(hitTolerance) {

src/ol/interaction/translate.js

@@ -212,7 +212,7 @@ ol.interaction.Translate.prototype.featuresAtPixel_ = function(pixel, map) {
 
 /**
  * Returns the Hit-detection tolerance.
- * @returns {number} Hit tolerance.
+ * @returns {number} Hit tolerance in pixels.
  * @api
  */
 ol.interaction.Translate.prototype.getHitTolerance = function() {
@@ -224,7 +224,7 @@ ol.interaction.Translate.prototype.getHitTolerance = function() {
  * Hit-detection tolerance. Pixels inside the radius around the given position
  * will be checked for features. This only works for the canvas renderer and
  * not for WebGL.
- * @param {number} hitTolerance Hit tolerance.
+ * @param {number} hitTolerance Hit tolerance in pixels.
  * @api
  */
 ol.interaction.Translate.prototype.setHitTolerance = function(hitTolerance) {

src/ol/render/canvas/replaygroup.js

@@ -71,6 +71,12 @@ ol.render.canvas.ReplayGroup = function(
    * @type {CanvasRenderingContext2D}
    */
   this.hitDetectionContext_ = ol.dom.createCanvasContext2D(1, 1);
+
+  /**
+   * @private
+   * @type {ol.Transform}
+   */
+  this.hitDetectionTransform_ = ol.transform.create();
 };
 ol.inherits(ol.render.canvas.ReplayGroup, ol.render.ReplayGroup);
 
@@ -174,7 +180,7 @@ ol.render.canvas.ReplayGroup.prototype.finish = function() {
  * @param {ol.Coordinate} coordinate Coordinate.
  * @param {number} resolution Resolution.
  * @param {number} rotation Rotation.
- * @param {number} hitTolerance Hit tolerance.
+ * @param {number} hitTolerance Hit tolerance in pixels.
  * @param {Object.<string, boolean>} skippedFeaturesHash Ids of features
  *     to skip.
  * @param {function((ol.Feature|ol.render.Feature)): T} callback Feature
@@ -187,15 +193,19 @@ ol.render.canvas.ReplayGroup.prototype.forEachFeatureAtCoordinate = function(
 
   hitTolerance = Math.round(hitTolerance);
   var contextSize = hitTolerance * 2 + 1;
-  var transform = ol.transform.compose(ol.transform.create(),
+  var transform = ol.transform.compose(this.hitDetectionTransform_,
       hitTolerance + 0.5, hitTolerance + 0.5,
       1 / resolution, -1 / resolution,
       -rotation,
       -coordinate[0], -coordinate[1]);
   var context = this.hitDetectionContext_;
+
+  if (context.canvas.width !== contextSize || context.canvas.height !== contextSize) {
     context.canvas.width = contextSize;
     context.canvas.height = contextSize;
+  } else {
     context.clearRect(0, 0, contextSize, contextSize);
+  }
 
   /**
    * @type {ol.Extent}

src/ol/renderer/layer.js

@@ -35,7 +35,7 @@ ol.inherits(ol.renderer.Layer, ol.Observable);
 /**
  * @param {ol.Coordinate} coordinate Coordinate.
  * @param {olx.FrameState} frameState Frame state.
- * @param {number} hitTolerance Hit tolerance.
+ * @param {number} hitTolerance Hit tolerance in pixels.
  * @param {function(this: S, (ol.Feature|ol.render.Feature), ol.layer.Layer): T}
  *     callback Feature callback.
  * @param {S} thisArg Value to use as `this` when executing `callback`.

src/ol/renderer/map.js

@@ -100,7 +100,7 @@ ol.renderer.Map.expireIconCache_ = function(map, frameState) {
 /**
  * @param {ol.Coordinate} coordinate Coordinate.
  * @param {olx.FrameState} frameState FrameState.
- * @param {number} hitTolerance Hit tolerance.
+ * @param {number} hitTolerance Hit tolerance in pixels.
  * @param {function(this: S, (ol.Feature|ol.render.Feature),
  *     ol.layer.Layer): T} callback Feature callback.
  * @param {S} thisArg Value to use as `this` when executing `callback`.
@@ -189,7 +189,7 @@ ol.renderer.Map.prototype.forEachLayerAtPixel = function(pixel, frameState, call
 /**
  * @param {ol.Coordinate} coordinate Coordinate.
  * @param {olx.FrameState} frameState FrameState.
- * @param {number} hitTolerance Hit tolerance.
+ * @param {number} hitTolerance Hit tolerance in pixels.
  * @param {function(this: U, ol.layer.Layer): boolean} layerFilter Layer filter
  *     function, only layers which are visible and for which this function
  *     returns `true` will be tested for features.  By default, all visible

src/ol/source/source.js

@@ -94,7 +94,7 @@ ol.source.Source.toAttributionsArray_ = function(attributionLike) {
  * @param {ol.Coordinate} coordinate Coordinate.
  * @param {number} resolution Resolution.
  * @param {number} rotation Rotation.
- * @param {number} hitTolerance Hit tolerance.
+ * @param {number} hitTolerance Hit tolerance in pixels.
  * @param {Object.<string, boolean>} skippedFeatureUids Skipped feature uids.
  * @param {function((ol.Feature|ol.render.Feature)): T} callback Feature
  *     callback.