Fix VectorLayer hitdetect inaccuracy

Due to rounding the hitdetection may have been off by one pixel.
At map edge the pixel coordinate may exceed the map's dimensions if an
decimal pixel ratio is used, this is fixed by clamping to the canvas
dimensions.

src/ol/render/canvas/hitdetect.js

@@ -6,6 +6,7 @@ import CanvasImmediateRenderer from './Immediate.js';
 import GeometryType from '../../geom/GeometryType.js';
 import IconAnchorUnits from '../../style/IconAnchorUnits.js';
 import {Icon} from '../../style.js';
+import {clamp} from '../../math.js';
 import {createCanvasContext2D} from '../../dom.js';
 import {intersects} from '../../extent.js';
 import {numberSafeCompareFunction} from '../../array.js';
@@ -162,8 +163,13 @@ export function createHitDetectionImageData(
 export function hitDetect(pixel, features, imageData) {
   const resultFeatures = [];
   if (imageData) {
+    // The pixel coordinate is clamped down to the hit-detect canvas' size to account
+    // for browsers returning coordinates slightly larger than the actual canvas size
+    // due to a non-integer pixel ratio.
     const index =
-      (Math.round(pixel[0] / 2) + Math.round(pixel[1] / 2) * imageData.width) *
+      (clamp(Math.floor(Math.round(pixel[0]) / 2), 0, imageData.width - 1) +
+        clamp(Math.floor(Math.round(pixel[1]) / 2), 0, imageData.height - 1) *
+          imageData.width) *
       4;
     const r = imageData.data[index];
     const g = imageData.data[index + 1];