Scrim

A macOS menu bar app that puts a transparent layer over every screen so you can draw on top of whatever is running (a slide, a document, a video call) and then get out of the way again. Nothing of it is on screen while you draw, it has no Dock icon, and it asks for no system permissions.

A scrim is the gauze a theatre hangs in front of a stage: paint on it and it holds the mark, light the stage behind it and it disappears. That is the whole idea.

macOS 11 or later · Apple Silicon and Intel · MIT licence · no permissions asked, no network access, no account.

What it does

• Draws over everything, including full-screen presentations: pen, marker, line, arrow, text, eraser, rectangle, oval, and a laser pointer.
• Steps aside in one gesture. Clicks, scrolls and keystrokes go back to the app underneath while the drawing stays on screen; the same gesture again puts the drawing away and keeps it.
• Costs nothing to leave running. No window, no Dock icon, and no CPU at all while the overlay is closed.

Installing

From a release. Download Scrim.zip from the Releases page, unzip it, and move the app to /Applications.

The first time you open it, macOS will refuse. The app is signed, but only ad-hoc signed (a local signature rather than a paid Developer ID), so you get "cannot be opened because Apple cannot check it for malicious software". You clear that once and never again:

• Right-click the app and choose Open, then Open again in the dialog. On macOS 15 and later this usually no longer works, so:
• open it normally, let it be refused, then go to System Settings → Privacy & Security and click Open Anyway.

From source. One command, no Xcode project, no dependencies, and no warning at all. You need Xcode or its command line tools (xcode-select --install):

git clone https://github.com/stackedd/scrim.git
cd scrim && ./build_app.sh && open dist/Scrim.app

Either way, a small scribble icon appears in the menu bar. That is the whole interface. On macOS 13 and later the menu has Open at Login, which starts the app with the Mac; on 11 and 12, add it under System Settings → General → Login Items.

The first sixty seconds

Six keys, in two rows under the left hand: A S D for what you draw with, Z X C for what happens to what you have drawn. Three of them open a wheel; three are a single press. Hold one, push the mouse, let go. The wheel opens right where the pointer is, and each slice is a 40° wedge, so you push in a direction rather than hitting a target. All five work whatever app is in front, because they are registered with the system rather than with a window.

Key	What it does
⌥A	Tools: pen, marker, line, arrow, text, eraser, rectangle, oval, laser, in that order round the wheel, starting on the right
⌥S	Size of the pen, the marker, the hole the eraser takes out, the laser's beam, or the type
⌥D	Colours
⌥Z	Undo. One press takes one thing back; hold it and it keeps going, like ⌘Z anywhere else
⌥X	What else to do to a drawing: redo, move, erase area, temporary ink, hide
⌥C	Clear. One press and the screen is empty again — ⌥Z puts it back, and the badge says so

⌥A works from the moment the app starts. The other five come and go with the overlay: while it is closed they are ordinary keys, and belong to whatever app you are using.

Two rules and that is the interface:

• The middle of a wheel is its default, and letting go there chooses it. On ⌥A the middle is the way out; on the other two it is a plain cancel.
• Tapping a wheel key does nothing, on purpose, so a key pressed by accident never changes the tool, the colour or the drawing.

⌃⌥⌘Esc quits the app from any state. It is the only other shortcut, it cannot be changed, and it always works.

If one of those keys is already taken by something else, open Settings… from the menu bar and press the keys you would rather use. Every shortcut needs ⌘, ⌥ or ⌃ in it. The same window sets how long each wheel waits before it appears - 110ms out of the box, and 0 opens it the moment you press the key.

(⌥ is Option, ⌃ is Control, ⌘ is Command.)

The tools

Tool	What it is
Pen	A plain line. Six colours, six widths.
Marker	Four times wider and see-through, like a highlighter over text.
Line, arrow, rectangle, oval	Two-point shapes. Holding Shift while dragging snaps to 45°, or makes a square or a circle.
Text	Click where you want it and type. Return finishes, Escape throws it away, clicking somewhere else finishes one and starts the next. The size wheel is the point size.
Eraser	Rubs out the part of a stroke it passes over: it cuts strokes rather than deleting them. Its size is the hole it leaves. Text is taken away whole - half a word is not a word.
Move	On the ⌥X wheel rather than with the drawing tools, because it changes what is already there. Drag anything you have drawn — a line, a shape, a word — and one undo puts it back. Picking it takes the screen back, the way picking any tool does.
Erase area	Also on ⌥X. Drag a box and what is inside it goes: lines are cut at its edge, words go whole. The whole box is one undo.
Laser	A glow that follows the pointer. Hold the button down and it leaves a beam of light that thins out behind your hand and is gone in about half a second. Nothing is left on the drawing.

Temporary ink, on the ⌥X wheel, makes every mark fade out a few seconds after it is finished, the way a presenter's pen does. Three seconds out of the box, and anything from 1 to 30 in Settings… — how long you need depends on how fast you talk. While it is on, the badge in the corner wears an orange TEMP, so a drawing that disappears is never a surprise.

Colour, width and tool are remembered between launches. The eraser and the laser are not: what comes back is the last tool that actually drew.

Three states, one gesture

• Drawing. The overlay takes the mouse and the keyboard. Nothing reaches the app underneath, and every key is swallowed, including Escape. The corner badge has a red stripe.
• Click-through. The drawing stays on screen, but clicks, scrolls and keystrokes go straight to the app below, so a slide can be advanced or a page scrolled without losing it. Letting go of the ⌥A wheel in the middle gets here; picking any tool takes the screen back.
• Away. Letting go in the middle once more. The overlay closes and the drawing is kept: picking any tool brings it back where it was, undo history and all.

⌥C is the only thing that erases a drawing, and even that can be undone.

If something goes wrong

The overlay is taking clicks and the menu bar is out of reach. Hold ⌥A and let go in the middle; that hands the screen back. If nothing responds at all, ⌃⌥⌘Esc quits the app.

The menu bar icon is not there. The app has no Dock icon and nothing on screen. When the menu bar is full macOS hides what does not fit, without telling the app or you. Scrim notices this a couple of seconds after it starts and says so on screen, with the keys you need; holding Command and dragging the menu bar icons makes room for it again.

⌥A does nothing. Another app may have registered the same shortcut; macOS allows that without reporting a conflict. The menu bar item does the same job, and the shortcuts are defined in one file for anyone building their own copy.

The pointer vanished during a presentation. It should not. While drawing, the pointer is painted on the overlay rather than handed to the system, precisely because presentation apps hide the system cursor. If it happens, it is a bug worth reporting.

Known limits

These follow from asking for no permissions, and they are not going to change:

• Drawings do not follow content. Scrolling a page leaves the ink where it was. Following content would need Accessibility.
• The Dock still reacts. Passing over it while drawing pops up app names, because macOS tells the Dock where the pointer is whatever window is on top.
• Nothing is saved. Drawings live in memory, survive hiding the overlay, and are gone when the app quits. There is no file to export or to lose.
• Changing the display arrangement while drawing closes the overlay rather than guessing where the strokes should go.

How it works

One transparent, non-activating panel per screen, at pop-up-menu window level so it sits above full-screen apps and on every Space. Ink is drawn into a CALayer and only the rectangle that changed is repainted. The global shortcuts use Carbon's RegisterEventHotKey, which needs no Accessibility permission. Open at Login uses SMAppService.

The frameworks are AppKit, Carbon, QuartzCore and ServiceManagement, with no third-party dependencies. The only thing written to disk is the small settings record (tool, colour, width) in the app's own preferences.

Checking that it works

Everything below runs from a clone, on any Mac:

./Testing/run.sh

• 137 behaviour checks drive the real app: every wheel and what its slices do, hiding and showing keeping the strokes and the undo history, the eraser cutting rather than deleting (and taking text away whole), the laser, temporary ink, typing, clearing and taking it back, the rules that keep a changed shortcut usable, and who owns the pointer in each state.
• A pixel comparison of one drawing painted incrementally and painted in a single pass, at 1x, 2x and 3x backing scale. They agree everywhere except 44 near-white pixels at one arrowhead, by at most 4 parts in 255 — antialiasing where two cut pieces of the same mark overlap, invisible at size, and the suite prints the box they sit in so that any movement shows. That is how "the optimisation did not change what is on screen" is proved rather than asserted.
• A cost suite that times painting: seven sweeps, including a single 5000-point stroke and a drag with the marker at its widest.

Measured on a real screen (Testing/probes/onscreen.swift), as a percentage of one core:

State	CPU
Overlay up, nothing moving	~0.9%
Pointer moving over 200 strokes	~1.7%
Drawing over 200 strokes	~5.9%
Overlay closed	0.0%

Taken 2026-09-02 on a 14-inch MacBook Pro (M3 Pro), macOS 26.6 - before the text tool and the current key layout, and not taken again since. Every number in docs/ARCHITECTURE.md carries a stamp like this one, so you can tell at a glance how old it is; the ones from ./Testing/run.sh are current, because anybody can take them again in a minute.

What the suites cannot check, and what it has been checked against instead. Nothing here can inject a real mouse or start a real slideshow, so the part this app exists for is checked by hand — and by hand means Keynote 13.2, full screen, on one 14-inch MacBook Pro running macOS 26.6. That is the only presentation app any of it has ever been tried with. PowerPoint, Google Slides in a browser, and screen sharing in Zoom or Teams have not been tried at all, and neither has a second display. If you use one of those, docs/RELEASE.md has the list of things to look at, and a report either way is worth more to this project than another number.

The permission claim is checkable in two commands rather than taken on trust:

grep -rn "AXUIElement\|CGEventTap\|ScreenCaptureKit\|URLSession" Sources/   # prints nothing
grep -c "UsageDescription" Packaging/Info.plist                            # 0

There is no network code, no analytics, and no telemetry of any kind.

Building and developing

swift build -c release      # the binary; must be warning-free
./build_app.sh              # the universal .app plus a zip, in dist/
./Testing/run.sh            # every suite

There is no XCTest target. Tests are built by compiling the app's own sources into a probe that drives the real code and prints what happened; Testing/README.md explains how, and how to add one. Several probes render PNGs (the wheels, the badge, the cursors, the laser, the ink) because some things can only be checked by looking at them.

The documentation is short and worth reading before changing anything:

• docs/ARCHITECTURE.md: what each file owns, the invariants, and every measurement.
• docs/DECISIONS.md: why things are as they are, including what was tried and dropped.
• docs/RELEASE.md: how a release is cut.
• CLAUDE.md: the operational summary, meaning commands, conventions, and the list of things never to do.

Issues and pull requests are welcome. Anything that touches drawing should come with ./Testing/run.sh output and a note on what moved.

Licence

MIT. See LICENSE. Use it, change it, ship it.