[OS X] Source Engine major bug with mouse input. Devs please read

Edit: I figured it out. Background application called BetterTouchTool causes mouse problems in several fps games and exiting it fixed these problemsThere appears to be a bug with source engine that affects all macOS/OS X players in all source engine games. I have reported this with the in-game report tool and also with steam support, with no results. (Steam support told me to report here). This makes all source games if not unplayable, at least very hard to play.The problem is that when left clicking with mouse, while simultaneously moving the mouse, the engine does not rotate your camera for half a second. After the delay, the engine rotates the camera to the desired rotation instantly, causing a twitching camera movement.What this does in practice is that it prevents aiming right after shooting, say for example a sniper rifle. Also shooting accurately with shotguns is a challenge.This affects mostly weapons that cant be fired while holding the mouse down, since the delay happens only with the initial mouse press, meaning the command bind to the key is not relevant. I have tried unbinding the mouse1, which did not affect the occurrence of the bug.I have also tried all combinations of mouse settings within the games. Also graphics settings such as vsync don't seem to affect this.The bug has been tested with these laptops:Macbook late 2016 (macOS Sierra)Macbook late 2008 (OS X Yosemite)In addition with this desktop:3.7 GHz Intel Core i3-43608 GB 1600 MHz DDR3NVIDIA GeForce GTX 750 Ti 2047 MBAlso tried with multiple mice and multiple trackpads. (Not that I would play with a trackpad)Source Engine games that I have confirmed have this bug:Team Fortress 2Garry's ModHalf Life 2Half Life 2 DeathmatchThank you for any help/advice, even though I am fairly certain this is a bug within the engine. As long as this bug exists, I unfortunately cannot play any source engine games.