How to Truly Full-Screen Window in macOS Stage Manager (Even with an External Keyboard!)

Have you ever settled in to work on your Mac, turned on Apple’s Stage Manager to keep your desktop organized, and tried to maximize a window—only to watch macOS stubbornly leave a giant empty strip on the left side of your screen?

If you use window-snapping apps like Rectangle, you’ve likely noticed the problem: hitting your maximize shortcut expands the window, but it stops short by about 15% to 20%, reserving precious real estate for Stage Manager’s app thumbnails.

Even worse, when you finally discover macOS’s built-in "Fill" shortcut (Fn + Control + F), it works like a dream on your MacBook keyboard—until you plug in a Bluetooth keyboard like the Logitech Pebble, and suddenly nothing happens at all.

Here is what is happening behind the scenes, why your external keyboard refuses to cooperate, and how to fix it in under two minutes.

1. The Real Issue: "Full Screen" vs. "Fill"

Before fixing the shortcut, it helps to understand how modern macOS treats window sizes:

The Fill command is what most people actually want: maximum screen space on the active desktop without isolating the app into its own virtual monitor space.

2. The Mystery: Why Fn + Control + F Breaks on External Keyboards

On your MacBook’s built-in keyboard, pressing Fn + Control + F immediately triggers "Fill."

So why does it fail on external keyboards like the Logitech Pebble (K380, Pebble Keys 2, or standard mechanical keyboards)?

• On a Mac keyboard: The Fn key is a native modifier recognized directly by macOS (acting as both Fn and the Globe key).
• On third-party keyboards: The Fn key is usually processed inside the keyboard itself to control hardware features like media volume, screen brightness, or device switching.

Because the keyboard never actually sends an Fn signal to macOS, your Mac only receives Control + F—and ignores the command.

3. The 2-Minute Fix: Make "Fill" Work Everywhere

You don't need to buy an Apple Magic Keyboard to fix this. You just need to assign the "Fill" command to a shortcut that doesn't use the Fn key.

Step 1: Open System Settings

1. Click the Apple logo () in the top-left corner of your screen.
2. Select System Settings.
3. In the sidebar, scroll down and click Keyboard, then click the Keyboard Shortcuts… button.

Step 2: Create a Custom Shortcut for "Fill"

1. In the window that pops up, select App Shortcuts on the left.
2. Click the + (plus) button at the bottom of the list.
3. Keep Application set to All Applications.
4. In the Menu Title field, type Fill exactly as shown (capital "F", no punctuation—it must match Apple’s menu bar item).
5. In the Keyboard Shortcut field, press a combination of standard modifier keys, such as:

6. Control + Option + F (Recommended)

7. Click Done.

Step 3: Put Your Logitech Pebble in "Mac Mode"

External keyboards often share layouts between Windows and macOS. To ensure the keys align correctly:

• Press and hold Fn + O on your Logitech keyboard for 3 seconds.
• The Bluetooth LED will flash, confirming the keyboard is now mapped to Apple modifier keys (Control, Option, and Command).

Test it out: Select any app window and press Control + Option + F on your Pebble keyboard. The window will immediately snap edge-to-edge, hiding the Stage Manager rail until your cursor touches the side of the display!

4. Bonus: Love the Rectangle App? Remove the Gap Permanently

If you already have strong muscle memory for Rectangle’s maximize shortcut, you don't even have to leave it behind. You can tell Rectangle to stop reserving space for Stage Manager:

1. Click the Rectangle icon in your menu bar and select Quit Rectangle.
2. Press Command + Space, type Terminal, and press Enter.
3. Copy and paste the following line, then press Enter:

defaults write com.knollsoft.Rectangle stageSize -float 0

1. Re-open Rectangle.

Now, when you hit your usual Rectangle shortcut, your window will stretch across 100% of your screen width automatically.