Casrion On Windows?
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Casrion, on a Mac.

For macOS 11 and later, Apple Silicon.

Two minutes of flipping switches, because macOS wants to be asked before an app can read what you highlighted. Then it lives in your menu bar and gets out of the way.

Download for Mac Free, no email, no account.
Apple Silicon only, for now.

Getting it open

Apple charges for a signature the same way Microsoft does, so macOS treats Casrion as a stranger exactly once. Here is the whole dance.

  1. Drag it into Applications

    Straight out of the .dmg, like anything else.

  2. Right click it, choose Open

    Then Open again in the dialog. Double clicking will not work the first time, and that is deliberate on Apple's part. Still refusing? Run xattr -cr /Applications/Casrion.app in Terminal once, then open it normally.

  3. Turn on Accessibility

    System Settings → Privacy & Security → Accessibility. This is what lets Casrion read the text you highlighted in another app. Without it, explain comes up empty. Casrion opens that pane for you the first time you ask it to explain something.

  4. Turn on Input Monitoring, if you want it

    Same place, one row down. macOS guards raw mouse taps separately from the switch above, which catches people out. All this one does now is let the explain popup notice you clicking away from it. Skip it and everything still works. The popup closes when you switch to another app, or with its own close button.

  5. Quit and reopen, properly

    From the menu bar icon, not by closing the window. macOS only applies those switches to a fresh launch. The microphone and browser questions show up on their own when you first need them; click Allow.

Every key

Mostly Cmd where Windows uses Ctrl. Three are not, because macOS keeps those combinations for itself and will not give them up. Those say so on the line. Inside the app, CmdShiftH puts this list on screen whenever you want it.

Copy something first, then press

Anything you can select and copy, in any app you have open.

Keep it as it is Cmd+Shift+C
Keep it as a big heading Cmd+Shift+1
Medium heading Cmd+Shift+2
Small heading Not Cmd+Shift+3. macOS takes a screenshot with that one. Cmd+Ctrl+3
Keep it as code Cmd+Shift+K
Keep it bold Cmd+Shift+B
Keep it italic Cmd+Shift+I
Keep it red, green or blue Not Option+R/G/B. On a Mac those type ® ˙ ∫. Cmd+Ctrl+R/G/B

Nothing to copy

These make something new rather than keeping something you found.

Drop in a screenshot Ctrl+Cmd+Shift+4 to snip. That Ctrl is what puts it on the clipboard instead of your desktop. Cmd+Shift+V
Start or stop a voice memo Cmd+Shift+M
Open a whiteboard and draw Cmd+Shift+D
Type a quick note from anywhere Not Cmd+Shift+Q. That one logs you out. Cmd+Shift+J
Start a new paragraph Cmd+Shift+N
Undo the last capture Cmd+Shift+Z
Redo it Cmd+Shift+Y

Explain a selection

Highlight first. No need to copy anything for this one.

Explain what I highlighted Two taps, quickly, without holding it down. Needs Input Monitoring. Cmd,Cmd
The same thing, and this one only needs Accessibility Cmd+Shift+E

Inside the whiteboard

Only while the board is open and in front of you.

Undo a stroke Cmd+Z
Put it back Cmd+Y
Close without saving Esc

The explain thing

Highlight anything, anywhere, press CmdShiftE, and a small window tells you what it says. It reads the screen around your selection first, so it knows a loose symbol belongs to the formula three lines up.

One honest exception: not in Safari. Safari keeps its page text to itself, so Casrion has nothing to read there. TextEdit, Notes, Preview, PDFs and Chrome all work.

The first time you use it, it downloads a small model, once. After that it works on a plane. Nothing you highlight is ever sent anywhere, because there is nowhere for it to be sent to.

When something acts up

It says Casrion cannot be opened.
Right click it in Applications and choose Open, then Open again. Double clicking will not do it. If it still refuses, xattr -cr /Applications/Casrion.app in Terminal once.
Explain says nothing is selected, but I selected something.
If you were in Safari, that is expected and there is no fix. Safari does not hand its page text to other apps through the accessibility system, so there is nothing for Casrion to read. It works in TextEdit, Notes, Preview, PDFs and Chrome. Anywhere else, Accessibility is either off or still switched on for an older copy of the app: turn it on, then quit Casrion from the menu bar and reopen it.
The explain popup will not go away when I click somewhere.
Same switch. Noticing a click outside its own window is exactly the thing Input Monitoring governs. Turn it on, then quit and reopen. Casrion spots this happening and offers you the settings pane from inside the popup.
I updated and a permission stopped working.
macOS ties permissions to the exact copy of the app, so the checkmarks can go stale while still looking switched on. Select Casrion in Accessibility and Input Monitoring, remove it with −, add it back with +, then quit and reopen.
Where do my notes live?
Markdown files in folders you picked, with screenshots, voice memos and whiteboards in an assets folder beside them. Nothing is in a database. They are just files.
Intel Mac?
Not yet. The build is Apple Silicon only for now.