WD19 Split

Two external monitors on a Mac that only supports one, through a Dell WD19 dock.

Base Apple silicon chips (M1, M2, …) have a single display engine for external screens, and macOS doesn't do DisplayPort MST. A Dell WD19 on those Macs just mirrors the same picture to every monitor. WD19 Split fixes that without drivers, kernel extensions or firmware changes:

• the Mac sends one wide picture (e.g. 3840×1080), which it can do;
• the dock's own Synaptics VMM5331 hub splits it across its two outputs, using a display mode the hub already supports (ViewXpand) but that Dell ships switched off;
• optionally, the app presents the two halves to macOS as two separate displays, each with its own Dock, menu bar, full-screen space and window snapping.

It is a small menu bar app plus a command-line tool, written in Swift and C.

Layouts

Layout	What macOS sees	Notes
Mirror	one monitor, copied to both screens	Dell's default
Ultrawide	one 3840×1080 display spanning both screens	zero added latency; works with protected video; left/right order is fixed by the dock (see below)
Two displays	two separate 1920×1080 displays	one frame of latency; Swap sides toggle; protected video shows black (by design)

Swapping sides. In Ultrawide, the dock decides which monitor gets the left half and which gets the right, based on the port each one is plugged into. The only way to swap them in this layout is to swap the monitor cables at the dock. (On the WD19 that can mean swapping an HDMI and a DisplayPort monitor, so you may need a matching cable or adapter.) Two displays has a software Swap sides toggle instead.

Every layout change shows a Keep / Revert prompt that reverts on its own after 15 seconds, like System Settings does for resolution changes.

Requirements

• A Mac on macOS 14 or later (developed on an M2 MacBook Pro, macOS 27).
• A Dell WD19 (Salomon family) whose MST hub runs firmware 05.06.03. The app checks the dock type, the hub firmware version and the hub's memory layout, and refuses anything it hasn't been verified against.
• Two monitors on the dock. Identical panels work best: the hub combines them side by side, and the combined width must stay under 4096 pixels.
• Xcode command-line tools (Swift 6) to build.

Other WD19 hub firmware versions, the WD22 and the newer WD25 family use different hub chips or layouts and are not supported.

Build and run

git clone https://github.com/kksimp/wd19-split.git
cd wd19-split
scripts/install_hooks.sh     # optional: secret-scan hooks for contributors
app/build_app.sh             # -> app/dist/WD19 Split.app and app/dist/dockctl
open "app/dist/WD19 Split.app"

build_app.sh signs with your first "Apple Development" identity if you have one (so macOS privacy permissions survive rebuilds), otherwise ad-hoc. Set SIGN_ID to choose another.

Using it

1. Open WD19 Split. Its window (and the menu bar icon, if macOS shows it) lists the dock, hub firmware and connected monitors.
2. Pick Ultrawide or Two displays.
3. The first time after the dock powers up, the window may ask you to unplug one monitor at the dock and plug it back in. The hub only rebuilds the display description it gives the Mac when a monitor is plugged in.
4. Two displays asks once for Screen Recording permission (System Settings → Privacy & Security → Screen & System Audio Recording). That is how it draws the two virtual displays onto the real signal.
5. Restore Dell defaults puts the dock back to mirroring.

Settings: open at login, re-apply your layout whenever the dock connects, and HiDPI for the ultrawide layout.

What it changes — and what it never does

• Changes: one byte in the hub's RAM (upstream display mode at 0x1740: 3 = Dell's mirror, 2 = split) and your Mac's display configuration. Unplugging the dock's power restores Dell's defaults completely.
• Never: writes the dock's firmware or flash, touches the hub's encrypted security / HDCP region, or disables or works around HDCP or any other content protection.
• Every hub write in the code goes through a single allowlisted function (dc_mst_set_edid_mode in app/Sources/DockCore).
• The two-display layout uses macOS's standard ScreenCaptureKit, which leaves protected content black. Use Ultrawide for streaming video.
• If the app quits or crashes, macOS removes its virtual displays by itself and the screens fall back to the ultrawide desktop.

How it works

Mac (one external display engine)
  └─ USB-C, DisplayPort alt mode, single stream ──► WD19 dock
                                                     └─ Synaptics VMM5331 MST hub
                                                          ├─ left half  ──► monitor 1
                                                          └─ right half ──► monitor 2

• Talking to the dock. The dock exposes a vendor HID device (413c:b06e, the RTS5487 hub's HID-to-I2C bridge). Through it the app reaches the dock's embedded controller and the MST hub, using the protocol documented by fwupd's dell-dock plugin. This is ordinary userspace IOKit HID: no driver, no SIP changes.
• The switch. The hub copies its config block to RAM at boot. Its EDID builder reads the low nibble of 0x1740: mode 3 passes the first monitor's EDID through (mirror), mode 2 builds a combined timing of monitor width × number of monitors and splits the video accordingly.
• The Mac side. macOS hides the unscaled 3840×1080 mode from its public API; the app selects it through the same window-server mode table System Settings uses. For two displays it creates two CGVirtualDisplays, captures them with ScreenCaptureKit, draws them side by side on a borderless overlay covering the real display, keeps the pointer on the virtual panels, and parks the real display where it doesn't steal the Dock or menu bar.

The full reverse-engineering notes (flash layout, MCS-251 source-mode disassembly, the EDID builder, live verification) are in research/PHASE1_FINDINGS.md. Firmware images, dumps and disassembly output are deliberately not in this repository; the Ghidra scripts and tools to reproduce them from Dell's public firmware are.

Command-line tool

app/dist/dockctl info              # dock model, versions
app/dist/dockctl mst-status        # hub bootloader status, active bank
app/dist/dockctl mst-mem 0x1740 4  # read hub RAM
app/dist/dockctl mst-poke 0x1740 2 # the one write: 2 = split, 3 = Dell default

Troubleshooting

• Log: ~/Library/Logs/WD19Split.log records every step of a layout change.
• Self-test: open -n "app/dist/WD19 Split.app" --args --selftest-two 15 turns the two-display layout on for 15 s without prompts, checks frames, pointer guard and Dock placement, writes pictures next to the log, then undoes everything.
• No menu bar icon: macOS hides menu bar items when the bar is full or the app isn't allowed there; opening the app again always shows its window.
• Clicking the desktop hides all windows: that's macOS's "Click wallpaper to reveal desktop" (System Settings → Desktop & Dock → set it to "Only in Stage Manager").
• Don't run Dell's dock firmware updater if you want to keep using this: a new hub firmware version is refused until it has been verified.

Project layout

app/Sources/DockCore       C library: HID bridge, dock EC and MST hub access
app/Sources/dockctl        command-line tool
app/Sources/WD19Split      SwiftUI app: hub control, display modes, two-display layout
app/Sources/VirtualDisplay declarations for CoreGraphics' virtual display classes
research/                  findings, Ghidra scripts, firmware-package extractor
scripts/                   secret scan and git hooks

Credits and disclaimer

The dock and hub protocol follows fwupd's dell-dock plugin (see NOTICE.md). This is an independent interoperability project, not affiliated with or endorsed by Dell, Synaptics or Apple. Use at your own risk; it only changes volatile settings, and a dock power cycle undoes them.

Licensed under the MIT License.