Sumbo

Mirror any window into a single always-on-top view — crop it, fade it, click through it.

Sumbo (숨보, from Korean "숨겨서 보자" — "keep it hidden, keep an eye on it") is a free, open-source Windows utility that live-mirrors any window using native DWM thumbnails. Keep a chat, a video, a dashboard, or a game map floating above your work — with zero impact on the source window (no capture, no injection, no performance cost to the mirrored app).

🇰🇷 한국어 안내는 README.ko.md

Screenshots

Interface shown in English with sample data.

Watch a video while you game — pin it on the left, with zero impact on the game window.

Features

• Single-window mirror — the main window is the mirror. Pick a target from the side rail and the whole canvas becomes a live view of it.
• Region crop — drag-select just the part you need; saved regions use relative coordinates, so they survive source-window resizing.
• Opacity 10–100% — see your work through the mirror.
• Click forwarding — click on the mirror, the click lands on the source window, no window switching.
• Click-through — the mirror ignores all input so you can work underneath it (escape with Ctrl+Alt+C).
• Size presets & anchoring — original / ½ / ¼ / fullscreen, 9-point screen anchors, position lock.
• Profiles — save target + region + opacity + placement, reapply in one click.
• Group switching — register several windows and cycle through them (Ctrl+Alt+G).
• Overlay mode (UI hide) — chrome disappears, only the mirrored content stays. The overlay window follows the crop's shape (no letterbox), and re-entering overlay restores the size and position you left it at.
• Tray resident — closing (X) hides to the tray (turn it off in Settings to make X exit for real); exit from the tray menu. Optional start-with-Windows.
• UI in 18 languages, switchable at runtime — English·한국어·日本語·简体中文· Español·Deutsch·Français·Português·Русский·Italiano·Bahasa Indonesia·Tiếng Việt· Türkçe·Polski·Nederlands·Українська·Ελληνικά·Filipino.

Download & install

Pick one from Releases:

Asset	Size	For
Sumbo-<version>-win-x64.msi	~38 MB	Installer — per-user (no admin/UAC), Start-menu shortcut, uninstall from Apps & Features. Runtime bundled.
Sumbo-<version>-win-x64.zip	~45 MB	Portable — unzip anywhere and run Sumbo.exe. Runtime bundled, nothing to install.
Sumbo-<version>-win-x64-lite.zip	~0.2 MB	Lite portable — same app without the bundled runtime. Requires the .NET 10 Desktop Runtime (Windows prompts with a download link on first run if missing).

Or via winget — no SmartScreen prompt on this route: winget install koprodev.Sumbo (catalog listing in review — available once approved)

(Optional) Verify a download: compare Get-FileHash <file> (PowerShell) against SHA256SUMS.txt attached to the release.

SmartScreen note: release binaries are not code-signed. On first run Windows may show "Windows protected your PC" — choose More info → Run anyway. If you prefer, build from source instead (below).

System requirements

• Windows 10 / 11, 64-bit (x64), with desktop composition (DWM) enabled — it is by default on all supported Windows versions.

Default hotkeys

Hotkey	Action
Ctrl+Alt+S	Show / hide the mirror window
Ctrl+Alt+W	Pick a target window
Ctrl+Alt+C	Toggle click-through
Ctrl+Alt+↑ / ↓	Opacity up / down
Ctrl+Alt+R	Select a region
Ctrl+Alt+G	Switch to the next window in the group

Privacy

• No telemetry, no accounts, no network access. Sumbo never phones home.
• Updates are manual — check the Releases page for new versions.

Known limitations

• Click forwarding cannot reach elevated (administrator) windows — Windows blocks input from a non-elevated process; Sumbo shows a one-time notice instead.
• Multi-monitor per-monitor-DPI transitions have had limited real-hardware verification. Reports are welcome via Issues.

FAQ · Tips

Q. I mirror a browser video (Chrome / Edge / Whale…), and it freezes whenever the browser is covered by another window.

That is the browser's power-saving occlusion detection, not Sumbo. Chromium-based browsers stop rendering when their window is completely covered by other opaque windows — and since Sumbo only re-composites what the source window draws (it never touches the source), the mirror freezes with it. This is also why the symptom seems to depend on which app is in front: transparent/layered windows don't count as occluders.

Fix: in the browser shortcut's properties, append the flag below to the Target field (after a space), then fully quit and restart the browser.

--disable-features=CalculateNativeWinOcclusion

Building from source

winget install Microsoft.DotNet.SDK.10   # then reopen the terminal
dotnet build -c Debug                    # build
dotnet test                              # unit tests
dotnet run --project src/Sumbo.App       # run

# Release single-file publish (what the release zip contains)
dotnet publish src/Sumbo.App -c Release -r win-x64 --self-contained `
  /p:PublishSingleFile=true /p:IncludeNativeLibrariesForSelfExtract=true `
  /p:DebugType=embedded -o artifacts/publish/win-x64-single

# Per-user MSI (what the release .msi contains) — WiX 5: dotnet tool install --global wix --version 5.0.2
wix build installer/Package.wxs -arch x64 -d Version=<version> `
  -d PublishDir=artifacts/publish/win-x64-single -d RepoDir=. `
  -o artifacts/publish/Sumbo-<version>-win-x64.msi

Project layout: src/Sumbo.App (WinForms UI) · src/Sumbo.Core (domain logic, UI-independent) · src/Sumbo.Native (P/Invoke wrappers for DWM/User32) · tests/Sumbo.Core.Tests (xUnit).

Support ☕

Sumbo is free and always will be — no ads, no feature gating. If it saves you some screen space and a few Alt+Tabs a day, consider supporting development:

github.com/sponsors/koprodev

License

MIT — see LICENSE. Release binaries bundle the .NET runtime; see THIRD-PARTY-NOTICES.md.

The window-mirroring approach was inspired by OnTopReplica; Sumbo is an independent, from-scratch implementation with no code reuse.