Linux Mint: Fix Unable to Type Equal Sign (=)

I've seen several other posts similar to this issue, but I've not had luck implementing a solution. If someone can find an existing post that resolves this issue, I will gladly mark this as a duplicate.

I'm not exactly sure on the timing, but perhaps within the past week or so, the equal key no longer types the equal sign (=). I have to copy and paste it to write this post.

The same symptoms extend to the onboard keyboard as well (not to be confused with the virtual keyboard which doesn't seem to have an equal key), so it's certainly not a hardware problem.

Shift + equal key still types the plus sign (+), but no key combination on the physical keyboard results in the equal sign (=) in web browsers and most other applications. The two exceptions with the physical keyboard I've found so far is with the GNOME Terminal. In the terminal, the combination of Ctrl or Windows key and equal key results in the equal sign. In addition, the Windows key and equal key types an equal sign in most text editors (LibreOffice Writer, Visual Studio Code, Xed text editor).

I can also use the equal key normally without a modifier key when in tty (Ctrl + Alt + F2). It's only within Cinnamon that I have the issue.

I only have one keyboard layout – English (US).

System Specs:

System:
  Host:       {HostName}
  Kernel:     5.3.0-28-generic x86_64
    bits:     64
    compiler: gcc
    v:        7.4.0 
  Desktop:    Cinnamon 4.4.8
    wm:       muffin
    dm:       LightDM
  Distro:     Linux Mint 19.3 Tricia 
    base:     Ubuntu 18.04 bionic 
Machine:
  Type:       Laptop
  System:     Acer
    product:  Aspire A717-72G
    v:        V1.19
    serial:   <filter> 
  Mobo:       CFL
    model:    Charizard_CFS
    v:        V1.19
    serial:   <filter>
  UEFI:       Insyde
    v:        1.19 
    date:     07/13/2018 

There was a kernel update in the past few weeks. I have avoided rolling back the kernel to the previous version because I didn't want to start grasping at straws and getting myself into worse trouble, so I thought I'd post this first.

Updates:

This is exactly the issue I'm having, but this issue is in Windows.

I have rebooted several times and I have checked multiple modifiers multiple times for stuck keys, but to no avail. I messed with this for days before finally deciding to post the issue. If it was as simple as a reboot or stuck key, I would expect to have seen that resolve by now.

Using xev the following is revealed (I'm new to xev and still learning what this means):

Equal Key:

KeymapNotify event, serial 28, synthetic NO, window 0x0,
keys:  66  0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   
       0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0

Example of All Other Keypresses I tried (i.e. Shift + equal key):

KeyPress event, serial 28, synthetic NO, window 0x7800001,
root 0x242, subw 0x0, time 11019227, (-650,-317), root:(211,139),
state 0x10, keycode 62 (keysym 0xffe2, Shift_R), same_screen YES,
XLookupString gives 0 bytes: 
XmbLookupString gives 0 bytes: 
XFilterEvent returns: False

KeyPress event, serial 28, synthetic NO, window 0x7800001,
root 0x242, subw 0x0, time 11019779, (-650,-317), root:(211,139),
state 0x11, keycode 21 (keysym 0x2b, plus), same_screen YES,
XLookupString gives 1 bytes: (2b) "+"
XmbLookupString gives 1 bytes: (2b) "+"
XFilterEvent returns: False

KeyRelease event, serial 28, synthetic NO, window 0x7800001,
root 0x242, subw 0x0, time 11019879, (-650,-317), root:(211,139),
state 0x11, keycode 21 (keysym 0x2b, plus), same_screen YES,
XLookupString gives 1 bytes: (2b) "+"
XFilterEvent returns: False

KeyRelease event, serial 28, synthetic NO, window 0x7800001,
root 0x242, subw 0x0, time 11020216, (-650,-317), root:(211,139),
state 0x11, keycode 62 (keysym 0xffe2, Shift_R), same_screen YES,
XLookupString gives 0 bytes: 
XFilterEvent returns: False

The equal keypress registers as a KeymapNotify event rather than a KeyPress event. When specifying xev -event keyboard, the equal keypress (KeymapNotify event) results in the same output as when my mouse cursor crosses the xed window.

Interestingly, using a modifier other than Shift (e.g. Windows/Super, Ctrl, or Alt) + equal key results in an xev response indicating the equal sign:

KeyPress event, serial 28, synthetic NO, window 0x7800001,
root 0x242, subw 0x0, time 11051562, (-650,-317), root:(211,139),
state 0x10, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
XLookupString gives 0 bytes: 
XmbLookupString gives 0 bytes: 
XFilterEvent returns: False

KeyPress event, serial 28, synthetic NO, window 0x7800001,
root 0x242, subw 0x0, time 11051836, (-650,-317), root:(211,139),
state 0x18, keycode 21 (keysym 0x3d, equal), same_screen YES,
XLookupString gives 1 bytes: (3d) "="
XmbLookupString gives 1 bytes: (3d) "="
XFilterEvent returns: False

KeyRelease event, serial 28, synthetic NO, window 0x7800001,
root 0x242, subw 0x0, time 11051930, (-650,-317), root:(211,139),
state 0x18, keycode 21 (keysym 0x3d, equal), same_screen YES,
XLookupString gives 1 bytes: (3d) "="
XFilterEvent returns: False

KeyRelease event, serial 28, synthetic NO, window 0x7800001,
root 0x242, subw 0x0, time 11052111, (-650,-317), root:(211,139),
state 0x18, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES,
XLookupString gives 0 bytes: 
XFilterEvent returns: False

Using xev | gawk '/keycode/{if($0!=l)print;l=$0;}' to filter out excess data, each keypress results in a line returned except for the equal key (thought Shift + equal key does – see lines 3 and 4).

state 0x10, keycode 36 (keysym 0xff0d, Return), same_screen YES,
state 0x10, keycode 20 (keysym 0x2d, minus), same_screen YES,
state 0x10, keycode 62 (keysym 0xffe2, Shift_R), same_screen YES,
state 0x11, keycode 21 (keysym 0x2b, plus), same_screen YES,
state 0x11, keycode 62 (keysym 0xffe2, Shift_R), same_screen YES,
state 0x10, keycode 37 (keysym 0xffe3, Control_L), same_screen YES,
state 0x14, keycode 37 (keysym 0xffe3, Control_L), same_screen YES,
state 0x10, keycode 37 (keysym 0xffe3, Control_L), same_screen YES,
state 0x14, keycode 54 (keysym 0x63, c), same_screen YES,
state 0x14, keycode 37 (keysym 0xffe3, Control_L), same_screen YES,

It would seem that the X server isn't seeing this keypress, though the keycode is mapped:

xmodmap -pke | grep equal results in keycode 21 = equal plus equal plus.

Additional info:

$ setxkbmap -query
  rules:      evdev
  model:      pc105
  layout:     us
  options:    terminate:ctrl_alt_bksp`