Description of Change

On macOS, entering or exiting fullscreen mode can fail even after windowWillEnterFullScreen or windowWillExitFullScreen has run (see the documentation for windowDidFailToEnterFullScreen and windowDidFailToExitFullScreen). If this happens, it is necessary to properly restore the window's original state, but Electron currently does not do that. The most prominent bug caused by this is that if the window fails to enter fullscreen, fullscreen_transition_state_ is "stuck" at FullScreenTransitionState::kEntering, and app.quit() does not work properly because the window refuses to close.

I don't know if it's possible to trigger these failures with automation, but windowDidFailToEnterFullScreen can be triggered manually on a Macbook without much effort:

1. Ensure the system has at least 2 virtual desktops (swipe up with 4 fingers and click the plus icon on the upper right).
2. Run the below snippet.
3. Before the first 3-second timer fires, press 3 or 4 fingers on the touchpad and move them slightly to one side (as if to swipe to another desktop). Keep the fingers held down, so that the UI remains in the "halfway" state between the two desktops without being fully in either one or the other.
4. Entering fullscreen is apparently not allowed when swiping between desktops like this, so windowWillEnterFullScreen runs followed by windowDidFailToEnterFullScreen. Without this change, the app will refuse to quit when the app.quit() call eventually runs. With this change, it quits normally.

const { app, BrowserWindow } = require('electron')

app.whenReady().then(() => {
  const mainWindow = new BrowserWindow({
    width: 800,
    height: 600,
    // Uncomment to test custom title bars, which require some special handling
    // titleBarStyle: "hidden",
    // trafficLightPosition: { x: 30, y: 30 },
  })

  mainWindow.loadFile("/dev/null");

  setTimeout(() => {
    mainWindow.setFullScreen(true)
    setTimeout(() => {
      mainWindow.setFullScreen(false)
      setTimeout(() => {
        app.quit()
      }, 3000)
    }, 3000)
  }, 3000)
})

Exiting fullscreen seems to behave oddly - when doing the same "swipe halfway into the other desktop" test with a window that is exiting fullscreen, windowDidFailToExitFullScreen does fire, but windowDidExitFullScreen also fires, and the window properly exits fullscreen once the user fully switches to a single desktop. To be safe, I tried to write this change such that it will work if either just windowDidFailToExitFullScreen runs or if both run.

Checklist

• PR description included and stakeholders cc'd
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Fix behavior when entering/exiting fullscreen fails on macOS.