macOS refuses to open it

Mindex builds are not yet signed with an Apple Developer certificate. macOS treats an unsigned app as untrusted.

The usual case

Right-click the app in Applications, choose Open, then Open again in the dialog. macOS asks once per build and then remembers.

Double-clicking will not offer that choice — the right-click is what produces the "Open anyway" dialog.

"Mindex is damaged and can't be opened"

This wording usually means the quarantine attribute is set on a download that macOS could not verify. Removing it:

xattr -dr com.apple.quarantine /Applications/Mindex.app

If it quits immediately on launch

A crash on launch with Code Signature Invalid means the build itself was packaged without a valid signature, rather than that macOS is being cautious. Redownload from the download page; if the current build still does it, please say so in Issues — that is a packaging fault, not something to work around on your machine.

Windows and Linux

Windows SmartScreen shows a similar warning: More info, then Run anyway. Linux has no equivalent step.