Displays have separate Spaces on a Mac: what the setting changes

A second display makes the desk larger and window management harder at the same time. Somewhere in that transition most people find a checkbox called "Displays have separate Spaces" and cannot tell whether switching it off will fix the annoyance they have or introduce three new ones. The checkbox is small and lives in one place, but it changes several unrelated behaviours together, which is why the advice about it online contradicts itself so badly. What follows separates those behaviours, states which of them Apple documents, and sets out how to decide based on the way the second display is actually used.

Where the setting lives and what Apple says it does

The option sits in System Settings, in the Desktop & Dock pane, inside the Mission Control group near the bottom of that pane. On recent versions of macOS the pane is long enough that the Mission Control group is below the fold, so it is easy to conclude the setting has been removed when it has only scrolled out of view.

Apple's description of the option is three sentences long, and every one of them is doing work.

Set up separate spaces for each display (if you use Spaces and have multiple displays). When this option is turned on, the Dock is available on all displays; when it's turned off, the Dock appears only on the main display. This option must be turned on if you want to use apps in Split View on your other displays or use Stage Manager. Source: support.apple.com

Three separate promises are packed into that. The first is about Spaces, the desktops created in Mission Control. The second is about where the Dock can appear. The third is a hard dependency: two named features stop being available when the option is off. Most of the frustration reported about this checkbox comes from people who changed it for the first reason and then ran into the second and third without connecting the events.

The same option is visible to software as well. Apple's developer documentation for AppKit exposes a property whose entire job is to report the state of this checkbox, described as reflecting "whether the 'Displays have separate Spaces' option is enabled in Mission Control system preference", and it exists so that an app can decide how to present itself in full screen. That is a useful thing to know before blaming an individual app for behaving differently after the setting changed. Some apps genuinely do read it and act on it.

What each display gets when the option is on

With the option on, each connected display carries its own set of Spaces. A display is no longer a window into one shared desktop. It is a stack of desktops of its own, and the stack it holds is independent of what the other display holds.

The practical consequences are easy to observe. Swiping between Spaces with three or four fingers, or pressing Control with the Right or Left arrow key, moves only the display the pointer is on. The other display holds still. Entering Mission Control behaves the same way, and Apple states the rule plainly: enter Mission Control on a second display and only the open windows and spaces being used on that display are shown. The Spaces bar along the top edge of the screen lists that display's desktops, not every desktop on the machine.

The Dock becomes reachable from either screen rather than living on one. macOS allows up to 16 Spaces, and with separate Spaces switched on those 16 are distributed across displays rather than shared.

There is a matching option in the Dock for pinning apps to a screen. Control-clicking an app icon in the Dock and opening Options reveals an Assign To list, and one of its entries is Desktop on Display followed by a number, which appears only when more than one display is available. That entry is the mechanism for saying a mail client belongs on the laptop screen and a code editor belongs on the large monitor, and it exists because displays are being treated as separate places.

What stops working when the option is off

Switching the option off merges the displays into a single space that happens to span two panels of glass. That produces the behaviour some people are looking for, because windows and full screen apps stop being confined to one screen. It also removes capabilities.

Apple documents the Dock consequence directly: with the option off, the Dock appears only on the main display. Reaching it from the secondary screen means dragging the pointer across the seam every time.

The two named dependencies are more consequential. Split View on the secondary displays requires the option to be on. Stage Manager requires it as well, and the failure mode there is unusually confusing: the Stage Manager control in Desktop & Dock settings is not hidden when the option is off, it is dimmed, and Apple's own reference for the pane tells the reader that a dimmed Stage Manager means the "Displays have separate Spaces" option needs turning on in the Mission Control section below. A dimmed control with no explanation next to it has sent a lot of people looking for a broken preference file.

One thing this setting does not govern is the menu bar. Which display carries the Finder and application menus is chosen in Displays settings, by picking the display and using the Use as pop-up menu. That is also where extending and mirroring are decided. Menu bar complaints are usually a Displays settings matter, not a Mission Control one.

The two states side by side

Behaviour	Option on	Option off
Spaces	Each display holds its own set	One set shared across all displays
Mission Control	Shows only that display's windows and spaces	Shows the joined desktop
Dock	Available on all displays	Appears only on the main display
Split View on other displays	Available	Not available
Stage Manager	Available	Control is dimmed
Assign To, Desktop on Display	Offered in the Dock options	Not meaningful
Which display has the menus	Set in Displays settings	Set in Displays settings

The table is short on purpose. Anything not listed is either unaffected or not documented, and the online arguments about this checkbox almost all take place in that undocumented space.

The other Mission Control options that get blamed for this one

Three neighbouring settings in the same group produce symptoms people attribute to separate Spaces, and checking them first saves a lot of switching back and forth.

"Automatically rearrange Spaces based on most recent use" reorders the desktops by recency. The result is that Control with an arrow key stops landing where muscle memory expects, because the order changed while the screen was not being watched. Anyone who navigates by keyboard rather than by looking at the Spaces bar usually wants this off.

"When switching to an application, switch to a Space with open windows for the application" makes the desktop scroll away to wherever an app already has a window. That is the cause of the classic complaint about a click on the Dock teleporting the user to a different screen or desktop. Apple gives a concrete illustration of it in the Spaces documentation: create a new TextEdit document on Desktop 3 while TextEdit windows are already open on Desktop 2, and the new document opens on Desktop 2.

"Group windows by application" only changes how Mission Control lays out thumbnails, and it is worth turning on before deciding that Mission Control is unusable with many windows open.

Moving around once the displays are separated

Separate Spaces multiplies the number of places a window can be, so the value of the setting depends on being able to reach those places quickly. macOS documents several routes, and most people use one of them and never learn the rest.

Mission Control opens with a three finger swipe up on a trackpad, a double tap with two fingers on a Magic Mouse, the Mission Control key, or Control with the Up arrow key. Dragging a window to the top of the screen also opens it, and that behaviour is itself a switch in Desktop & Dock settings that can be turned off if it keeps firing by accident. The gesture can be changed or disabled in Trackpad settings, and the keyboard shortcut in Keyboard settings.

Control with the Down arrow key shows every window belonging to the current app, which is the fastest way to find one browser window among nine. Command with the Mission Control key pushes all windows aside to reveal the desktop, and pressing it again brings them back. On a trackpad, spreading the thumb and three fingers apart does the same thing, and pinching them together reverses it.

Hot Corners, in the same Mission Control group, assigns an action to each corner of the screen, and a modifier key can be required so that the corner does not trigger when the pointer merely passes through. With two displays and separate Spaces, corners become genuinely useful, because there are now four corners per screen and the pointer is crossing them constantly anyway.

Deciding by what the second display is for

The decision gets simple once the question changes from which setting is better to what the second display is holding.

If the second display holds one thing for long stretches, a dashboard, a video call, a reference document, separate Spaces on is the state that matches. Full screen and Split View work there, Stage Manager is available, and the main display keeps swiping between desktops without disturbing it.

If the second display is used as extra area for one wide task, a timeline, a spreadsheet that needs the width, a window deliberately straddling the seam, then off is the state that matches, and the Dock and Split View limitations are the price.

There is a third case, and it is the most common one among people who work all day in browser based tools. The second display is not holding a task at all. It is holding a pile of web apps that need to stay visible: a chat client, a calendar, a mail account, a ticket queue. Neither state of this checkbox helps much there, because the problem is not how macOS groups desktops. The problem is that all of those tools live inside one browser as tabs, so the display is showing one window that constantly changes what it displays. Giving each of those tools a window of its own is a different layer of the same problem, and it is what an app aggregation browser is for. A tool built that way keeps each service in its own persistent window, which then makes the Assign To options in the Dock useful, because there is finally something stable to assign. The trade-offs between products in that category are laid out in Compared with Wavebox and Compared with Shift, and the way sets of apps get grouped per screen is covered under Workspaces.

What to change first

Turn the option on, leave it on, and turn off "Automatically rearrange Spaces based on most recent use" in the same visit, since that pair accounts for most of the disorientation blamed on separate Spaces. Then, if the second display is still cycling through browser tabs rather than showing stable windows, the layer to change is the browser rather than Mission Control, and SpaceDeck is built for that layer.