5 files changed, 23 insertions, 12 deletions
diff --git a/ext/bg/js/settings/keyboard-mouse-input-field.js b/ext/bg/js/settings/keyboard-mouse-input-field.js
index f9c6dfcd..d5a2a2ea 100644
--- a/ext/bg/js/settings/keyboard-mouse-input-field.js
+++ b/ext/bg/js/settings/keyboard-mouse-input-field.js
@@ -147,7 +147,7 @@ class KeyboardMouseInputField extends EventDispatcher {
     }
 
     _getModifierKeys(e) {
-        const modifiers = DocumentUtil.getActiveModifiers(e);
+        const modifiers = new Set(DocumentUtil.getActiveModifiers(e));
         // https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/metaKey
         // https://askubuntu.com/questions/567731/why-is-shift-alt-being-mapped-to-meta
         // It works with mouse events on some platforms, so try to determine if metaKey is pressed.
diff --git a/ext/fg/js/frontend.js b/ext/fg/js/frontend.js
index af1a6527..0ed842f5 100644
--- a/ext/fg/js/frontend.js
+++ b/ext/fg/js/frontend.js
@@ -250,6 +250,7 @@ class Frontend {
     }
 
     async _onActiveModifiersChanged({modifiers}) {
+        modifiers = new Set(modifiers);
         if (areSetsEqual(modifiers, this._activeModifiers)) { return; }
         this._activeModifiers = modifiers;
         if (this._popup !== null && await this._popup.isVisible()) {
diff --git a/ext/mixed/js/display.js b/ext/mixed/js/display.js
index 39bd7732..9824c25e 100644
--- a/ext/mixed/js/display.js
+++ b/ext/mixed/js/display.js
@@ -205,7 +205,7 @@ class Display extends EventDispatcher {
 
         const eventModifiers = DocumentUtil.getActiveModifiers(e);
         for (const {modifiers, action} of handlers) {
-            if (getSetDifference(modifiers, eventModifiers).size !== 0) { continue; }
+            if (!this._areSame(modifiers, eventModifiers)) { continue; }
 
             const actionHandler = this._actions.get(action);
             if (typeof actionHandler === 'undefined') { continue; }
@@ -1476,4 +1476,14 @@ class Display extends EventDispatcher {
         const {expression, reading} = termDetailsList[Math.max(0, bestIndex)];
         return {expression, reading};
     }
+
+    _areSame(set, array) {
+        if (set.size !== array.size) { return false; }
+        for (const value of array) {
+            if (!set.has(value)) {
+                return false;
+            }
+        }
+        return true;
+    }
 }
diff --git a/ext/mixed/js/document-util.js b/ext/mixed/js/document-util.js
index 90add8f9..da27a75d 100644
--- a/ext/mixed/js/document-util.js
+++ b/ext/mixed/js/document-util.js
@@ -170,11 +170,11 @@ class DocumentUtil {
     }
 
     static getActiveModifiers(event) {
-        const modifiers = new Set();
-        if (event.altKey) { modifiers.add('alt'); }
-        if (event.ctrlKey) { modifiers.add('ctrl'); }
-        if (event.metaKey) { modifiers.add('meta'); }
-        if (event.shiftKey) { modifiers.add('shift'); }
+        const modifiers = [];
+        if (event.altKey) { modifiers.push('alt'); }
+        if (event.ctrlKey) { modifiers.push('ctrl'); }
+        if (event.metaKey) { modifiers.push('meta'); }
+        if (event.shiftKey) { modifiers.push('shift'); }
         return modifiers;
     }
 
@@ -185,7 +185,7 @@ class DocumentUtil {
     }
 
     static getActiveButtons(event) {
-        const buttons = new Set();
+        const buttons = [];
         this._getActiveButtons(event, buttons);
         return buttons;
     }
@@ -301,19 +301,18 @@ class DocumentUtil {
         return !(browser === 'firefox' || browser === 'firefox-mobile') || os === 'mac';
     }
 
-    static _getActiveButtons(event, set) {
+    static _getActiveButtons(event, array) {
         let {buttons} = event;
         if (typeof buttons === 'number' && buttons > 0) {
             for (let i = 0; i < 6; ++i) {
                 const buttonFlag = (1 << i);
                 if ((buttons & buttonFlag) !== 0) {
-                    set.add(`mouse${i}`);
+                    array.push(`mouse${i}`);
                     buttons &= ~buttonFlag;
                     if (buttons === 0) { break; }
                 }
             }
         }
-        return set;
     }
 
     // Private
diff --git a/ext/mixed/js/text-scanner.js b/ext/mixed/js/text-scanner.js
index f4b8df68..189afd07 100644
--- a/ext/mixed/js/text-scanner.js
+++ b/ext/mixed/js/text-scanner.js
@@ -817,11 +817,12 @@ class TextScanner extends EventDispatcher {
 
     _getMatchingInputGroup(modifiers, type) {
         let fallback = null;
+        const modifiersSet = new Set(modifiers);
         for (let i = 0, ii = this._inputs.length; i < ii; ++i) {
             const input = this._inputs[i];
             const {include, exclude, types} = input;
             if (!types.has(type)) { continue; }
-            if (this._setHasAll(modifiers, include) && (exclude.length === 0 || !this._setHasAll(modifiers, exclude))) {
+            if (this._setHasAll(modifiersSet, include) && (exclude.length === 0 || !this._setHasAll(modifiersSet, exclude))) {
                 if (include.length > 0) {
                     return {index: i, empty: false, input};
                 } else if (fallback === null) {