How can I run iPhone simulator over full-screen(lion) Xcode 4.1?

To run simulators in full-screen mode, you need to enable fullscreen mode by executing the following command in your terminal:

defaults write com.apple.iphonesimulator AllowFullscreenMode -bool YES

*You need to restart the simulator in order to reflect the changes.

As others point out, Apple doesn't allow to run two full screen apps one over the other one. However, you could put them next to each other like this which already is an enhancement (at least for me):

I use VS Code instead of Xcode but the logic is the same. All you have to do is just to allow iOS Simulator to run in Full Screen by running the following command in terminal:

defaults write com.apple.iphonesimulator AllowFullscreenMode -bool YES

Just so you know, you can now do this as of Xcode 9. You need to enable the Internal menu in iOS Simulator.

To do this:

1. Create the folder AppleInternal in the root directory: sudo mkdir /AppleInternal

2. Restart all instances of the iOS Simulator.

3. Observe the available Internal menu, navigate to Internal | Allow Fullscreen Mode and ensure this option is ticked.

After following these steps, you can enable fullscreen mode on the iOS simulator.

Information taken from: https://medium.com/flawless-app-stories/simulator-on-steroids-c12774ca6b

I recommend going with BetterSnapTool or something similar. Then you can easily size the window to take up the full screen without losing the use of a second monitor or the ability to see Xcode and the simulator as you're debugging.

1. Shut down your Mac,
2. Boot into the Recovery Mode, When shut, start pressing CMD + R and while still pressing power up your machine and keep pressing the button combination until the Apple logo appears so that your Mac boots into the Recovery Mode.
3. Click Utilites and choose Terminal
4. Type diskutil list and identify the id of your disk named Macintosh HD, it should be something like /dev/disk2s1
5. Now we need to mount it: type diskutil mount /dev/[YOUR_DISK_ID]. If successful, keep going with the next step. If not, you probably got a message regarding APFS that the disk needs to be unlocked, so type diskutil apfs unlockVolume /dev/[YOUR_DISK_ID]
6. Finally you can create the required folder to enable hidden Xcode features: type mkdir /Volumes/[YOUR_MAC_DRIVE_NAME]/AppleInternal Now you can get out of the Recovery Mode and boot your Mac as usual. Open and run your project on simulator. Now you can see a new bar menu item named Internal. There is an option to set simulator to fullscreen.

In macOS Big Sur this is now supported, so you just go full screen with the Simulator window or drag it into split view as you would with any other window.