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