emscripten-core/emscripten

Bug: emscripten_set_wheel_callback is passive

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#10,091 opened on Dec 20, 2019

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 (6 comments) (1 reaction) (0 assignees)C++ (3,519 forks)batch import
good first bughelp wanted

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Description

Repro:

#include <emscripten/emscripten.h>
#include <emscripten/html5.h>

int callback(int a, const EmscriptenWheelEvent* b, void* c) {
  return EM_TRUE; // Call preventDefault() on the event
}

int main() {
  emscripten_set_wheel_callback(0, 0, 0, callback);
  return 0;
}

Compile the above with 1.38.43-upstream and embed in a HTML file with <script src="a.out.js"></script>. Open the HTML file and scroll with the mouse wheel.

Unexpected: Chrome prints out this to the console:

[Intervention] Unable to preventDefault inside passive event listener due to target being treated as passive. See https://www.chromestatus.com/features/6662647093133312

The fix is to add {passive: false} to the call to addEventListener like this:

window.addEventListener("wheel", callback, {passive: false});

Ideally this would be configurable in the emscripten_set_wheel_callback API call, but I don't see a way to do that.

See https://developers.google.com/web/updates/2017/01/scrolling-intervention for more information.

Contributor guide

Research direction
Add {passive: false} to the wheel event listener in emscripten's HTML5 event handling code.
Tech stack
ccppjavascriptwasm
Domain
tooling
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
1
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Under 1 hour
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C++Emscripten
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70