Why macOS Cannot Extend Displays with MST Hubs

When Mac users connect multiple monitors to a DisplayPort Multi-Stream Transport (MST) hub, macOS mirrors the displays instead of extending them into independent desktops. This behavior occurs because Apple’s operating system lacks driver-level support for DisplayPort MST, defaulting instead to Single-Stream Transport (SST). Consequently, macOS transmits only one video stream over the cable, which the hardware hub duplicates across every connected screen.

Understanding DisplayPort MST vs. SST

DisplayPort 1.2 introduced Multi-Stream Transport (MST), a protocol that allows a computer to route multiple independent video streams through a single DisplayPort or USB-C connection. With MST, a host system can assign distinct framebuffers to different monitors daisy-chained together or connected through a multi-port hub.

In contrast, Single-Stream Transport (SST) only supports transmitting a single video signal over one physical connection. Operating systems like Windows and Linux include built-in drivers for both SST and MST, allowing seamless desktop expansion over budget-friendly DisplayPort hubs.

Why macOS Restricts MST

Apple has chosen not to implement DisplayPort MST for external displays in macOS. This is an intentional software and driver architecture decision rather than a fundamental hardware limitation of Apple Silicon or Intel chips.

Because macOS only operates in SST mode over standard DisplayPort and USB-C alternate-mode connections:

  1. macOS identifies the hub as a single external display interface.
  2. The graphics subsystem generates and outputs only one continuous video feed.
  3. The MST hub receives this single stream and distributes identical copies to all connected monitors.

From the hub's perspective, it has no secondary or tertiary streams to route to the other screens, leaving hardware mirroring as the only possible fallback.

Solutions for Extending Displays on a Mac

To achieve multiple extended desktops on a Mac, users must bypass standard DisplayPort MST hubs using alternative technologies:

  • Thunderbolt 3, Thunderbolt 4, or USB4 Docks: Unlike standard DisplayPort hubs, certified Thunderbolt docks use high-bandwidth PCIe tunneling to carry two completely separate native DisplayPort streams over a single cable. macOS natively supports this architecture, allowing two extended displays (provided the Mac hardware supports multiple external monitors).
  • Direct Cable Connections: Connecting each display directly to a dedicated port on the Mac provides independent video signals for each monitor.
  • DisplayLink Adapters: For Macs limited to a single external display (such as base-model M1, M2, and M3 MacBooks), USB-based DisplayLink docks convert video data into standard USB data packets via proprietary software drivers, circumventing the native display pipeline to drive multiple extended screens.