move mouse position getter to lib and work-around Firefox bug (#5525)

Author: LucaVazz

src/lib/index.js

@@ -715,6 +715,18 @@ lib.isIOS = function() {
     return IS_IOS_REGEX.test(window.navigator.userAgent);
 };
 
+var FIREFOX_VERSION_REGEX = /Firefox\/(\d+)\.\d+/;
+lib.getFirefoxVersion = function() {
+    var match = FIREFOX_VERSION_REGEX.exec(window.navigator.userAgent);
+    if(match && match.length === 2) {
+        var versionInt = parseInt(match[1]);
+        if(!Number.isNaN(versionInt)) {
+            return versionInt;
+        }
+    }
+    return null;
+};
+
 lib.isD3Selection = function(obj) {
     return obj instanceof d3.selection;
 };
@@ -1263,3 +1275,28 @@ lib.join2 = function(arr, mainSeparator, lastSeparator) {
     }
     return arr.join(mainSeparator);
 };
+
+/**
+ * Return the mouse position from the last event registered by D3.
+ * @returns An array with two numbers, representing the x and y coordinates of the mouse pointer
+ *   at the event relative to the targeted node.
+ */
+lib.getPositionFromD3Event = function() {
+    var firefoxVersion = lib.getFirefoxVersion();
+
+    // see https://bugzilla.mozilla.org/show_bug.cgi?id=1684973
+    var isProblematicFirefox = firefoxVersion && firefoxVersion < 86;
+
+    if(isProblematicFirefox) {
+        // layerX and layerY are non-standard, so we only fallback to them when we have to:
+        return [
+            d3.event.layerX,
+            d3.event.layerY
+        ];
+    } else {
+        return [
+            d3.event.offsetX,
+            d3.event.offsetY
+        ];
+    }
+};

src/plots/geo/geo.js

@@ -506,11 +506,7 @@ proto.updateFx = function(fullLayout, geoLayout) {
     }
 
     bgRect.on('mousemove', function() {
-        var d3EventPosition = [
-            d3.event.offsetX,
-            d3.event.offsetY,
-        ];
-        var lonlat = _this.projection.invert(d3EventPosition);
+        var lonlat = _this.projection.invert(Lib.getPositionFromD3Event());
 
         if(!lonlat || isNaN(lonlat[0]) || isNaN(lonlat[1])) {
             return dragElement.unhover(gd, d3.event);