From 7f03da9588841e8d3da74e0dfeed50f95f2e4e6f Mon Sep 17 00:00:00 2001
From: Vittorio Romeo <[EMAIL REDACTED]>
Date: Tue, 5 May 2026 01:54:29 +0200
Subject: [PATCH] emscripten: dedupe keyboard event listeners across multiple
windows
---
src/video/emscripten/SDL_emscriptenevents.c | 22 +++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/src/video/emscripten/SDL_emscriptenevents.c b/src/video/emscripten/SDL_emscriptenevents.c
index d4bd854a9e41a..dcdee7914c971 100644
--- a/src/video/emscripten/SDL_emscriptenevents.c
+++ b/src/video/emscripten/SDL_emscriptenevents.c
@@ -1341,11 +1341,33 @@ 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"))