📱 TabScreen

Turn an old Android tablet into a second display for your Mac. About 33 ms of lag over Wi-Fi. No subscription, no cables, no developer mode.

spacedesk only ships a Windows server. Sidecar only talks to iPads. Duet wants a yearly fee. If you have a MacBook and an Android tablet gathering dust in a drawer, this is the missing piece.

Install

Two downloads from the Releases page. No terminal, nothing to build.

1. Install BetterDisplay on the Mac — the free version is enough. It creates the virtual screen; macOS has no public API for that, so no app can do it alone.

2. Unpack TabScreen-mac.zip and drag TabScreen.app to Applications. Open it with right-click → Open the first time — the app isn't signed with a paid Apple certificate, so a plain double-click gets blocked. Allow screen recording when macOS asks.

3. Press "Create virtual screen." TabScreen sets one up through BetterDisplay, 1280×800, ready to go.

4. On the tablet, open tabscreen-android.apk — download it straight to the tablet with its browser. Android will ask to allow installing from this source. That's the whole setup on that side: no developer mode, no adb, no USB.

5. Press "Turn on." The tablet finds the Mac on the network by itself. If your network blocks that, the app shows its address and a QR code — type it in on the tablet.

Now drag any window onto the new screen and it appears on the tablet.

The virtual screen comes up when you turn the stream on and goes away when you stop it, so nothing is left hanging around collecting windows.

Rotation

Four positions — 0°, 90°, 180°, 270° — set on the Mac, next to the screen picker. Handy when the tablet stands upright in a holder.

The tablet's own decoder does the turning while it draws, which costs nothing, and it obeys the Mac: there is no setting on that side, and picking the tablet up and turning it over changes nothing.

What to expect

Measured on a MacBook Air M1 and a Samsung Galaxy Tab A 10.1 from 2019 — 2 GB of RAM, a decoder from that era. Newer hardware does better; nearly all the remaining delay is the tablet decoding the picture.

Lag	31–36 ms
Frame rate	45–60 fps
Mac CPU	2 % of one core
Network	~5 Mbit/s

For comparison: the obvious way to build this — screen recording piped into a browser — gives 200–500 ms and feels awful. How it got to 33 ms.

When something goes wrong

"The app won't open, macOS says it's damaged or from an unidentified developer." Right-click the app → Open → Open. This is macOS's standard treatment of apps without a paid certificate, not a sign of anything wrong.

The tablet says "Looking for a Mac" forever. Both devices need to be on the same Wi-Fi, and guest networks usually block devices from seeing each other. Type the address shown in the TabScreen window instead.

The tablet shows a black screen. The Mac is capturing the wrong display — check the "Screen" dropdown, the virtual one is labelled Virtual. If there's no virtual screen in the list, press "Create virtual screen".

"Screen recording is off for this app", but the switch in System Settings is on. The app is signed ad-hoc, so every rebuild changes its signature and the old permission stops matching while still looking granted. Clear it and grant it again:

tccutil reset ScreenCapture com.tabscreen.mac

Then open TabScreen and allow it. Do not press "Deny" in that dialog — that writes a refusal, and re-flipping the switch afterwards won't override it; you'd have to run the reset again.

"BetterDisplay isn't responding." Start BetterDisplay by hand and wait until its icon appears in the menu bar, then try again.

The picture is smooth but lags behind. Something else is eating your Wi-Fi, or the tablet is far from the router. A USB cable is an option for the stubborn cases — see dev/ — but it needs developer mode on the tablet and buys only a few milliseconds.

The tablet itself is sluggish, not the picture. Old Android tablets spend their RAM on preinstalled software. tools/debloat.sh turns off the usual suspects — nothing is deleted, tools/rebloat.sh puts it all back. That one does need adb.

Building it yourself

Only if you want to change something.

git clone https://github.com/BreraDMR/tabscreen
cd tabscreen
./setup.sh     # builds the Mac app, fetches a JDK + Android SDK into ./tools, builds the APK

Nothing is installed system-wide and no password is asked for. The Android client is ~450 lines of Java, the Mac app ~700 lines of Swift, and the APK comes out at 20 KB.

• How it works and why it's fast — the architecture, the four fixes that mattered, and the measurements
• KNOWN-GOOD.md — every setting of the configuration that produced these numbers, plus what was tried and made things worse
• dev/ — earlier Python and ffmpeg iterations, kept for debugging

Limits

• Touch isn't wired up. There's a working implementation in dev/input.py, but at any noticeable lag a finger-driven cursor feels wrong.
• Rotation is set on the Mac only - by design, so the tablet can be handled without the picture flipping around.
• BetterDisplay is required for the virtual screen. No way around it on macOS.
• The app is unsigned — first launch needs right-click → Open.
• macOS 13+ and Android 8+.

License

Licensed under PolyForm Noncommercial 1.0.0 — free for personal, educational, and other noncommercial use. Commercial use requires a separate license; contact damir.brera.eb@gmail.com.