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" & "Factory reset all connected Apple devices" & "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 & 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 & Click tab → and saw that the "Look up & 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 & 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.

In my case, I had placed an external keyboard on the built-in trackpad. That was messing up the mouse clicks.