Adding Hotkey.allowOverlap flag (to allow "conflicts"/overlap of hotk…

…eys), see #1621

js/input/Hotkey.ts

@@ -59,7 +59,9 @@ type SelfOptions = {
   // Fire continuously at this interval (milliseconds)
   fireOnHoldCustomInterval?: number;
 
-  // TODO: consider attach:false https://github.com/phetsims/scenery/issues/1621
+  // For each main `key`, the hotkey system will only allow one hotkey with allowOverlap:false to be active at any time.
+  // This is provided to allow multiple hotkeys with the same keys to fire. Default is false.
+  allowOverlap?: boolean;
 };
 
 export type HotkeyOptions = SelfOptions & EnabledComponentOptions;
@@ -74,6 +76,7 @@ export default class Hotkey extends EnabledComponent {
   public readonly fireOnDown: boolean;
   public readonly fireOnHold: boolean;
   public readonly fireOnHoldTiming: HotkeyFireOnHoldTiming;
+  public readonly allowOverlap: boolean;
 
   // A Property that tracks whether the hotkey is currently pressed.
   // Will be true if it meets the following conditions:
@@ -106,7 +109,8 @@ export default class Hotkey extends EnabledComponent {
       fireOnHold: false,
       fireOnHoldTiming: 'browser',
       fireOnHoldCustomDelay: 400,
-      fireOnHoldCustomInterval: 100
+      fireOnHoldCustomInterval: 100,
+      allowOverlap: false
     }, providedOptions );
 
     super( options );
@@ -119,6 +123,7 @@ export default class Hotkey extends EnabledComponent {
     this.fireOnDown = options.fireOnDown;
     this.fireOnHold = options.fireOnHold;
     this.fireOnHoldTiming = options.fireOnHoldTiming;
+    this.allowOverlap = options.allowOverlap;
 
     // Create a timer to handle the optional fire-on-hold feature.
     if ( this.fireOnHold && this.fireOnHoldTiming === 'custom' ) {

js/input/hotkeyManager.ts

@@ -184,12 +184,12 @@ class HotkeyManager {
    * 2. All modifier keys in the hotkey's modifierKeys pressed
    * 3. All modifier keys not in the hotkey's modifierKeys (but in the other hotkeys above) not pressed
    */
-  private getHotkeyForMainKey( mainKey: EnglishKey ): Hotkey | null {
+  private getHotkeysForMainKey( mainKey: EnglishKey ): Hotkey[] {
     const englishKeysDown = this.englishKeysDownProperty.value;
 
     // If the main key isn't down, there's no way it could be active
     if ( !englishKeysDown.has( mainKey ) ) {
-      return null;
+      return [];
     }
 
     const compatibleKeys = [ ...this.enabledHotkeysProperty.value ].filter( hotkey => {
@@ -205,9 +205,13 @@ class HotkeyManager {
       } );
     } );
 
-    assert && assert( compatibleKeys.length < 2, `Key conflict detected: ${compatibleKeys.map( hotkey => hotkey.getHotkeyString() )}` );
+    if ( assert ) {
+      const conflictingKeys = compatibleKeys.filter( hotkey => !hotkey.allowOverlap );
 
-    return compatibleKeys[ 0 ] ?? null;
+      assert && assert( conflictingKeys.length < 2, `Key conflict detected: ${conflictingKeys.map( hotkey => hotkey.getHotkeyString() )}` );
+    }
+
+    return compatibleKeys;
   }
 
   /**
@@ -216,7 +220,7 @@ class HotkeyManager {
    */
   private updateHotkeyStatus(): void {
     for ( const hotkey of this.enabledHotkeysProperty.value ) {
-      const shouldBeActive = this.getHotkeyForMainKey( hotkey.key ) === hotkey;
+      const shouldBeActive = this.getHotkeysForMainKey( hotkey.key ).includes( hotkey );
       const isActive = this.activeHotkeys.has( hotkey );
 
       if ( shouldBeActive && !isActive ) {
@@ -271,8 +275,8 @@ class HotkeyManager {
 
       this.englishKeysDownProperty.value = new Set( [ ...this.englishKeysDownProperty.value, englishKey ] );
 
-      const hotkey = this.getHotkeyForMainKey( englishKey );
-      if ( hotkey ) {
+      const hotkeys = this.getHotkeysForMainKey( englishKey );
+      for ( const hotkey of hotkeys ) {
         this.addActiveHotkey( hotkey, keyboardEvent, true );
       }
     }
@@ -282,8 +286,8 @@ class HotkeyManager {
 
   private onKeyUp( englishKey: EnglishKey, keyboardEvent: KeyboardEvent ): void {
 
-    const hotkey = this.getHotkeyForMainKey( englishKey );
-    if ( hotkey ) {
+    const hotkeys = this.getHotkeysForMainKey( englishKey );
+    for ( const hotkey of hotkeys ) {
       this.removeActiveHotkey( hotkey, keyboardEvent, true );
     }