How a non-developer fixed two bugs in a Mac app built with AI

samuel Jeong — Wed, 11 Nov 2026 01:46:11 +0000

I never learned programming, but I did not want my 24-inch M1 iMac sitting idle at home, so I started building an app with AI coding tools like Claude to use it as a secondary monitor for my MacBook. When I shared the first beta version in early November, I ran into an unexpected situation. I only owned one 24-inch M1 machine, but 11 out of the 14 people who applied were using 27-inch 5K Intel iMacs. I had to continue testing without access to the actual hardware.

A user with a late 2015 27-inch 5K iMac and an M1 MacBook Pro reported the detailed behavior. First, when connecting the cable, display settings detected two separate screens. Looking into the cause, the discovery routine for the second iMac was finding the same machine again over its wired address, which created an extra virtual display. I changed the second connection step to first request the unique installation ID sent by the iMac, and if it matches the first ID or fails to respond, no display is generated.

Another issue was that unplugging the cable crashed the app on the iMac side. When the connection dropped, the main thread released the decoder while the receiving thread was still handling decoding tasks at that exact moment, leading to a crash. I added a lock to the decoder, stopped closing the socket immediately during disconnect handling in favor of a shutdown call, and let each receiving thread handle its own exit. Before the fix, disconnecting the cable caused a crash in all three tests. After the change, it stayed stable in all three tests.

Because I did not have an Intel iMac, I could not test the fixed build right away, but that user ran tests on their hardware that night. After plugging and unplugging the cable, they shared that "the app runs very smoothly without any crashes" and confirmed the duplicate screen issue was gone. After bringing up 4.5K resolution, they also noted that "the screen looks much cleaner and better to look at."

Many people are already using it now. If you are interested in using an iMac as a second monitor, you can try the beta version for 3 days at the address below and consider purchasing it if you like it.

https://videofactories.com/submonitor/?ref=devto

I built SubMonitor Pro to turn an unused 24" Apple Silicon iMac into a wired 4.5K 60Hz monitor over 1 cable

samuel Jeong — Sat, 07 Nov 2026 14:30:21 +0000

If you have an unused 24-inch Apple Silicon iMac sitting on your desk at home, Apple removed hardware Target Display Mode years ago—leaving wireless AirPlay lag, $89.99 hardware dongles, or recurring $36–$72/yr subscriptions as the usual workarounds.

I built SubMonitor Pro (v0.4.0, Build 13), a lightweight native Swift + AppKit/SwiftUI macOS utility that turns an idle 24-inch iMac into a crisp wired secondary display for your MacBook over a single Thunderbolt or USB-C cable.

Core Specifications & Native App Window Controls

1-Cable Offline Link: Uses Thunderbolt / USB-C offline IPv6 link-local networking (~0.4 ms RTT, zero Wi-Fi lag or internet requirement).
Dual Display Modes:
- Native 4.5K 1:1 (4480×2520 @ 60Hz) for razor-sharp Retina text and code
- Lightweight QHD+ (2560×1440 @ 60Hz) to save bandwidth on standard USB-C cables
Single Native Control Window (Menu Bar Popover & Dashboard):
- Display Brightness slider + global keyboard shortcuts (⌃⌥- / ⌃⌥=)
- Built-in 6-Speaker 48kHz PCM audio toggle & volume slider
- Interactive Left / Right / Top / Bottom arrangement map (auto-restores on reconnect)
macOS Sleep Sync: Observes NSWorkspace.willSleepNotification and screensDidSleepNotification on your host Mac so the iMac panel enters dark standby automatically and wakes right back up with your Mac.
8 Built-in Languages: English, Korean, Japanese, Simplified/Traditional Chinese, German, Spanish, and French.

18.5-Second Demo Video & Free Beta Waitlist

18.5s Fact-Only Demo Video + Live Waitlist: https://videofactories.com/submonitor/?lang=en&ref=devto
Direct MP4 Demo: https://videofactories.com/submonitor/assets/demo-en.mp4
Pricing Plan: No $89.99 dongle and $0/mo recurring subscription—planned release price is a flat $29 USD one-time lifetime license (confirmed at beta exit). Free beta invites are open now for MacBook + iMac owners!

DEV Community: samuel Jeong

How a non-developer fixed two bugs in a Mac app built with AI

I built SubMonitor Pro to turn an unused 24" Apple Silicon iMac into a wired 4.5K 60Hz monitor over 1 cable

Core Specifications & Native App Window Controls

18.5-Second Demo Video & Free Beta Waitlist