emscripten: dedupe keyboard event listeners across multiple windows

src/video/emscripten/SDL_emscriptenevents.c

@@ -1327,11 +1327,33 @@ void Emscripten_RegisterEventHandlers(SDL_WindowData *data)
 
     keyElement = Emscripten_GetKeyboardTargetElement(data->keyboard_element);
     if (keyElement) {
+        // Emscripten's HTML5 helpers do not deduplicate `addEventListener` calls:
+        // see `registerOrRemoveHandler` in `emscripten/src/lib/libhtml5.js`.
+        //
+        // If a previous SDL window already registered keyboard handlers on the same
+        // target, the new registration would *stack* a second listener, causing every
+        // browser keydown to fire `Emscripten_HandleKey` twice.
+        //
+        // The duplicate calls then produce two `SDL_EVENT_KEY_DOWN` events per physical
+        // keypress (the second one with `repeat=true`, due to the keystate-based repeat
+        // detection in `SDL_SendKeyboardKeyInternal`).
+        //
+        // We must clear any prior handler on this target before installing ours:
+        emscripten_set_keydown_callback(keyElement, NULL, 0, NULL);
+        emscripten_set_keyup_callback(keyElement, NULL, 0, NULL);
+        emscripten_set_keypress_callback(keyElement, NULL, 0, NULL);
+
         MAIN_THREAD_EM_ASM_INT({
             var data = $0;
             // our keymod state can get confused in various ways (changed capslock when browser didn't have focus, etc), and you can't query the current
             //  state from the DOM, outside of a keyboard event, so catch keypresses globally and reset mod state if it's unexpectedly wrong. Best we can do.
             //  Note that this thing _only_ adjusts the lock keys if necessary; the real SDL keypress handling happens elsewhere.
+
+            // Remove any prior listener first -- `addEventListener` does not deduplicate either.
+            if (document.sdlEventHandlerLockKeysCheck) {
+                document.removeEventListener("keydown", document.sdlEventHandlerLockKeysCheck);
+            }
+
             document.sdlEventHandlerLockKeysCheck = function(event) {
                 // don't try to adjust the state on the actual lock key presses; the normal key handler will catch that and adjust.
                 if ((event.key != "CapsLock") && (event.key != "NumLock") && (event.key != "ScrollLock"))