Mac often perceives left-click on Magic Mouse as right-click The bug:

All of a sudden, my 2013 rMBP often perceives a left-click on my Magic Mouse 2 as a right-click.

Details:


"Often" ≈ every 30 seconds.
I have 2 different mouses: an Apple Magic Mouse 1 and an Apple Magic Mouse 2. The problem persists when using either mouse. (Only one is switched on at a time.)
The right-click still functions properly.
The built-in trackpad does not have this issue. It still functions properly.
I've read that the problem might be somehow related to Google Chrome. But, the problem persists (e.g., in Finder) even when Google Chrome is not running.
I've terminated the keyboard-remapping software, "Karabiner." I do not have the purportedly-problematic apps, "BetterTouchTool" or "Magnet," installed.
No peripherals are plugged into the computer.
As soon as I notice that the left-clicks are right-clicks, I do not touch the mouse for about 5 seconds, and the mouse goes "back to normal"... until it happens again (and the process repeats).
I plugged in a wired USB Dell mouse (model # 0CJ339). The wired mouse does not have the issue.


What I've tried:


I've disconnected and deleted my wireless keyboard as a Bluetooth device (to ensure that no other Bluetooth device is connected to my rMBP other than my mouse).
Turning Bluetooth on and off.
 shift  +  ⌥ option  + left-clicking the Bluetooth menu bar icon → Debug → "Reset the Bluetooth module" &amp; "Factory reset all connected Apple devices" &amp; "Remove all devices"
Switching mouse on and off. Recharging mouse.
System Preferences → Mouse → toggling the "Secondary Click" checkbox and ensuring that "Click on right side" is selected


I've discovered another bug that could be related: When "Secondary click" is set to "Click on left side" in System Preferences → Mouse, the computer never perceives a right-click (no matter whether the mouse is left-clicked or right-clicked). It behaves as if the "Secondary click" checkbox is not even enabled.

Deleting Apple Bluetooth and mouse preference files found in ~/Library/Preferences/
Resetting PRAM &amp; SMC
Booting in Safe Mode. (The issue still manifests in Safe Mode.)
Logging out of the administrator account and logging into the "Guest" user account (where the issue still manifested).
Repeatedly pressing the  ⌃ control  key on the laptop keyboard to ensure that a small piece of foreign matter is not stuck underneath this key. (When I hold down the  ⌃ control  key and left-click, the rMBP perceives this as a right-click 100% of the time.) Also, opening "Keyboard Viewer" to ensure that my computer does not perceive any modifier keys as being pressed when the issue occurs.
I do not have any other lingering Bluetooth devices in the vicinity that might interfere with the mouse.


Is there anything else that I can try, besides reinstalling the OS?



OS X El Capitan, version 10.11.6.
Update:

I randomly noticed that a three-finger-tap on the built-in trackpad would not pull up the dictionary entry for the selected word (like it used to).

I navigated to:


System Preferences → Trackpad → Point &amp; Click tab → and saw that the "Look up &amp; data detectors" checkbox was not enabled. 


This was very strange, because this feature has always been enabled, and I never disabled it. I enabled this checkbox, and the "look-up" feature now worked.

But, this immediately made my computer stop perceiving all right-clicks (i.e., right-clicks on my mouse were always perceived as left-clicks).

So, I navigated to:


System Preferences → Mouse → Point &amp; Click tab → and toggled the "Secondary click" checkbox off and back on.


After doing this, my computer has not perceived a left-click as a right-click again.

I still have no clue what caused the issue, or why doing the above solved it, especially considering that, for good measure, I had already previously deleted the com.apple.AppleMultitouchTrackpad.plist and com.apple.driver.AppleBluetoothMultitouch.trackpad.plist preference files.