Left click of MacBook trackpad not registering with external 4K monitor connected

Describe the bug
When an external 4K monitor is connected, around 30% of left clicks from the trackpad aren't registered. Taps from the trackpad or clicks from an external mouse work just fine.
I completely reset my BTT settings and the problem persists. Currently, the only solution is to disable BTT ("for active app" is enough). Since it affects the normal left click, it's really annoying and I can't work with an external monitor and BTT at the same time.
I've had this problem for at least a few months, so it's not related to recent updates.

Affected input device (e.g. MacBook Trackpad, Magic Mouse/Trackpad, Touch Bar, etc.):
MacBook Trackpad

If applicable, add screenshots to help explain your problem. (You can just paste or drag them here)

Device information:

  • Type of Mac: MacBook Pro Touch Bar 15 (A1707, 2016)
  • macOS version: 10.13.6
  • BetterTouchTool version: 2.692
  • Monitor: LG 31MU97Z-B (Thunderbolt 2, Cinema 4K 40962160 instead of regular UHD 38402160)

Additional information (e.g. StackTraces, related issues, screenshots, workarounds, etc.):

Note: Before bug reporting, please make sure you have the latest version of BetterTouchTool and that you have already tried to restart your system :-). If you encounter a crash, please attach a crash log from the macOS Console.app from the "User Diagnostic Reports" section.

do you have any usb devices attached to that monitor? Similar issues can happen if a defective USB device is constantly disconnecting/reconnecting.

The monitor has an integrated USB hub, but you have to connect a dedicated USB cable to the MacBook in order to use it (it wasn't connected to the hub). I still just disconnected all USB devices from the monitor but the problem persists.
On my MacBook there are no other devices connected other than the official TB3 to TB2 adapter. Disconnecting the charger didn't help, either.

I just noticed that other gestures, especially swipe gestures, doesn't work as consistent either, when connected to the monitor. Maybe something internal in BTT gets slowed down quite heavily while being connected to the monitor? When checking Activity Monitor, idle % CPU of BTT is constantly at 1 to 5% instead of 0.1 to 2%.

Sounds a lot like it's similar to the defective USB issue I described. The constant connect/disconnect notifications would also cause an increase in CPU usage and slow things down.

Could you go to the about tab in BTT and click the "Export Debug Information" button right after you reproduced the issue? This will create a folder called BetterTouchToolDebug on your desktop. If you send that to andreas@folivora.ai I'll take a look at the logs, maybe they show what's going on.

Okay, just sent the mail.

Just exported another one right after restarting BTT and making a few gestures and left clicks, if that helps.

Unfortunately the logs show what I thought. A constantly (multiple times per second) disconnecting and reconnecting device.

Currently I have no solution to this, as BTT relies on the USB connect & disconnect notifications macOS sends in order to discover newly connected devices. In the future I may switch to a different approach though.
However this will not only increase CPU usage in BTT, but also system CPU usage, as the system constantly thinks a USB device has connected and disconnected.

Maybe it's somehow caused by the integrated USB hub? Does it maybe need to be powered externally or something like that? So far I only had two users with similar issues in the last few years, but in their case it was a defective USB stick, which was easier to replace...

Oh okay thanks, then I'll try to find out how to fix the monitor. Sorry for bothering you with the bug report.

Please let me know if you can find anything that solves this!

I will try to add some sort of workaround to BTT which will prevent looking for new devices if too many connect/disconnect notifications happen

Restarting the monitor a few times until no USB error shows up in Console.app helped for now. Even though that's no real fix, now at least I know It's not BTT's fault and I have a workaround.