electron/docs/tutorial/planned-breaking-changes.md

Planned Breaking API Changes (4.0)

The following list includes the APIs that will be removed in Electron 4.0.

There is no timetable for when this release will occur but deprecation warnings will be added at least one major version beforehand.

webFrame

// Deprecated
webFrame.registerURLSchemeAsPrivileged('app', {secure: true})
// Replace with
protocol.registerStandardSchemes(['app'], {secure: true})

Nota Bene: Before we can remove this we need to update all of the relevant specs to protocol.registerStandardSchemes(['app'], {secure: true}).

app.makeSingleInstance

// Deprecated
app.makeSingleInstance(function (argv, cwd) {

})
// Replace with
app.requestSingleInstanceLock()
app.on('second-instance', function (argv, cwd) {

})

app.releaseSingleInstance

// Deprecated
app.releaseSingleInstance()
// Replace with
app.releaseSingleInstanceLock()

FIXME comments

The FIXME string is used in code comments to denote things that should be fixed for the 3.0 release. See https://github.com/electron/electron/search?q=fixme