Initial release: patched Electron builds fixing WebSocket starvation
Fixes a Chromium regression where continuous mouse input (e.g., shooting in FPS games) starves WebSocket/Worker message dispatch when --disable-frame-rate-limit is active. Includes: - Patch diff for main_thread_scheduler_impl.cc - Automated CDP stress test for verification - Full build-from-source guide Pre-built binaries available in Releases. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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# Electron WebSocket Fix
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Patched Electron builds that fix a Chromium regression where continuous mouse input starves WebSocket and Worker message dispatch when `--disable-frame-rate-limit` is active.
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This is critical for competitive browser-based games (like [Krunker](https://krunker.io)) running in Electron, where shooting (holding left click + moving mouse) causes network freezes of 100-300ms+.
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## Downloads
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Pre-built patched binaries (Windows x64):
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| File | Electron Version | Size |
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|------|-----------------|------|
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| [`electron-v40.6.1-release-patched-win32-x64.zip`](releases/electron-v40.6.1-release-patched-win32-x64.zip) | v40.6.1 (latest stable) | 133MB |
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| [`electron-v42.0.0-nightly-release-patched-win32-x64.zip`](releases/electron-v42.0.0-nightly-release-patched-win32-x64.zip) | v42.0.0-nightly | 137MB |
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Both are full release builds (`is_official_build = true`) with maximum optimizations.
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## The Problem
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When an Electron app uses these flags (common for competitive gaming):
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```javascript
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app.commandLine.appendSwitch('disable-frame-rate-limit');
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app.commandLine.appendSwitch('disable-gpu-vsync');
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```
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...continuous mouse input with left-click held down causes WebSocket `onmessage` callbacks to be delayed by 100-300ms+. This manifests as network "freezing" -- player positions stop updating, hit registration breaks, and the game becomes unplayable during gunfights.
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### Root Cause
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Three factors in Chromium's Blink main thread scheduler combine to create the issue:
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1. **Input tasks at `kHighestPriority`** (priority level 1) while WebSocket/Worker tasks sit at `kNormalPriority` (level 7)
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2. **No cross-priority anti-starvation** in `task_queue_selector.cc` -- it always picks from the highest active priority queue, with no mechanism to let lower-priority tasks run
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3. **Compositor priority boost during input** -- when mouse is held + moving (`UseCase::kMainThreadCustomInputHandling`), the compositor queue gets boosted to `kHighestPriority`. With `--disable-frame-rate-limit`, the `BackToBackBeginFrameSource` posts `SEND_BEGIN_MAIN_FRAME` tasks at zero delay, creating an infinite loop of highest-priority tasks that permanently starves everything else
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This regression was introduced in Chromium 84 when `PrioritizeCompositingAfterInput` was made unconditional ([CL 2132022](https://chromium-review.googlesource.com/c/chromium/src/+/2132022)).
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## The Fix
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Two changes in one file (`main_thread_scheduler_impl.cc`):
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1. **Lower input task priority** from `kHighestPriority` to `kNormalPriority`
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2. **Cap compositor priority** to `kNormalPriority` via `std::max()`
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See [`patches/ws-priority-patch.diff`](patches/ws-priority-patch.diff) for the exact diff.
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### Test Results
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12-second automated stress test with continuous CDP mouse input (left-click held, circular movement):
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| Build | p99 Latency | Max Latency | Messages >50ms | Mouse Events | Frames |
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|-------|-------------|-------------|----------------|-------------|--------|
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| **Unpatched** | ~97ms | ~308ms | 8.6% | ~11,380 | ~6,300 |
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| **Patched** | ~34ms | ~38ms | **0%** | ~12,360 | ~7,620 |
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The patch not only eliminates starvation but actually **improves** both input throughput (+9% mouse events) and frame rate (+21% frames) because it prevents the input/compositor priority cascade from monopolizing the main thread.
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## Usage
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### Option A: Direct Binary
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Extract the zip and run your app:
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```bash
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electron.exe path/to/your/app
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```
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### Option B: Replace in node_modules
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```bash
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# Back up original
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mv node_modules/electron/dist node_modules/electron/dist-original
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# Extract patched version
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mkdir node_modules/electron/dist
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cd node_modules/electron/dist
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unzip path/to/electron-v40.6.1-release-patched-win32-x64.zip
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```
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### Option C: electron-builder
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In `package.json`:
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```json
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{
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"build": {
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"electronDist": "path/to/extracted/dist",
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"electronVersion": "40.6.1"
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}
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}
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```
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### Option D: electron-forge
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In `forge.config.js`:
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```js
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module.exports = {
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packagerConfig: {
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electronZipDir: 'path/to/extracted/dist'
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}
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};
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```
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## Verification
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An automated stress test is included in the [`test/`](test/) directory:
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```bash
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cd test
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npm install
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path/to/patched/electron.exe cdp-test.js 8085 PATCHED
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```
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The test uses CDP `Input.dispatchMouseEvent` to simulate continuous mouse input (the only reliable automated method -- Electron's `sendInputEvent` API bypasses the compositor thread and doesn't trigger the bug).
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Expected output for a patched build: **0% of WebSocket messages >50ms**.
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## Building From Source
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Full build instructions are in [`BUILD-GUIDE.md`](BUILD-GUIDE.md).
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Quick summary:
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```bash
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# 1. Set up environment (depot_tools, VS Build Tools, Python, etc.)
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# 2. Initialize and sync Electron source
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e init --root=C:\electron my-build --import release
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e sync
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# 3. Check out desired version
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cd src/electron && git checkout v40.6.1
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cd .. && gclient sync --with_branch_heads --with_tags
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# 4. Apply patch
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git apply path/to/ws-priority-patch.diff
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# 5. Configure and build
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mkdir -p out/Release
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# Set out/Release/args.gn:
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# import("//electron/build/args/release.gn")
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# is_official_build = true
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# use_remoteexec = false
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# use_reclient = false
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buildtools/win/gn.exe gen out/Release
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ninja -C out/Release electron
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ninja -C out/Release electron:electron_dist_zip
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```
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Expect 6-10+ hours for a full build on a modern machine (24 cores, 64GB RAM).
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## Patch Details
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The patch modifies Chromium source (not Electron source), so it applies to any Electron version. Line numbers may shift between versions but the function names remain the same:
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- `ComputePriority()` -- search for `PrioritisationType::kInput`
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- `ComputeCompositorPriority()` -- search for that function name
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File: `third_party/blink/renderer/platform/scheduler/main_thread/main_thread_scheduler_impl.cc`
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## License
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The patch itself is provided as-is. Electron is MIT licensed. Chromium is BSD licensed.
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