macos - The left modifier keys on my Macbook Pro has a delay before working after wake up

Occasionally, my mid-2014 Macbook Pro 15 Retina's left modifier keys (Shift, Option, Control) don't work immediately after resuming from sleep.

For example:

• My login password requires pressing the Shift key. I have to use the right-Shift or Caps Lock, unless I wait 20 seconds or so for the left-Shift key to wake up.




• Immediately after Sleep, if I shift-select text in a text editor such as Atom or the web browser's URL bar, pressing the arrow keys while holding shift will simply move the cursor for awhile. Sometimes only 5 characters, sometimes 30 characters, and then the shift-selection highlight will start working.

Curiously, left-shift starts working within a short space of time (less than a minute), every time. And once it's working, it will stay working while the Mac is on. It always starts working again within a minute.

When it starts working it is 100% working - in other words, it's not like I have to press the key extra hard or anything. When it's working, I can press gently anywhere on the left-shift key and it will perform the function correctly.

Putting the MBP into sleep and then waking it, often but not every time, triggers the "sleepy left-shift" problem. The longer the Mac has been in sleep the higher chance it will have the delay before working, although sometimes a brief sleep causes it too, it is somewhat unpredictable.
This makes it slow to test for resolution, as it can appear like the problem is fixed.

I've tried:

• PRAM and SMC resets




• Removed a number of plist preferences




• Checked accessibility options including Sticky and Slow Keys. I've turned these on and then off.




• Disconnected my Apple Bluetooth keyboard in case that was interfering. The problem still exists.




• Turned on the keyboard viewer to check if any keys were stuck and causing the left-shift not to register. No keys appeared stuck.

Any ideas please? I'm nearly at the point of doing a clean install, but I would really like to solve the problem.

It is possible it's a hardware fault which would be annoying. But it seems somewhat unlikely given the problem goes away every time, and only affects the left-shift key. The flag for it being hardware is that my Apple Bluetooth keyboard doesn't have the problem, so it's not likely to be a software hook causing the problem, unless that is only applied to the internal keyboard.

OS X 10.11.4.

Answer

Solution: I had the keyboard replaced. Now the Macbook is working fine. Bonus is that the keyboard feels better, like firmer.