I realize this is probably not a KDE Neon specific question, but that's the flavor I'm running, so I'm posting here.

For approximately the last 6 months or so, I have noticed a change in the behavior of pressing shift and space together. I do this accidentally when I'm typing fast and recently the behavior has changed, but only in some programs.

In most programs shift+space results in a space, which is my desired output. However, in Chromium and Thunderbird, it does nothing. I've checked both of those program's shortcut lists, and shift+space isn't mapped to anything. Firefox, LibreOffice, Kate, Konsole, Dolphin, etc. all work the way I want.

Any ideas to fix or troubleshoot this behavior?

Hi,
Could you send the output of the following command?
inxi -c 5 -b
The terminal output on my computer is the following:
inxi -c 5 -b
System: Host: jochoapc Kernel: 4.13.0-45-generic x86_64 (64 bit) Desktop: KDE Plasma 5.13.4
Distro: neon 16.04 xenial
Machine: Mobo: ASRock model: G965M-S Bios: American Megatrends v: P1.20 date: 05/12/2009
CPU: Dual core Intel Pentium Dual E2200 (-MCP-) speed/max: 1726/2203 MHz
Graphics: Card: Intel 82G965 Integrated Graphics Controller
Display Server: X.Org 1.19.6 drivers: (unloaded: fbdev,vesa) Resolution: 1280x1024@60.02hz
GLX Renderer: Mesa DRI Intel 965G GLX Version: 2.1 Mesa 18.0.5
Network: Card-1: Realtek RTL8101/2/6E PCI Express Fast/Gigabit Ethernet controller driver: r8169
Card-2: Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller driver: r8169
Drives: HDD Total Size: 750.2GB (22.9% used)
Info: Processes: 148 Uptime: 3:53 Memory: 1508.6/3245.4MB Client: Shell (bash) inxi: 2.2.35

I did what you say but nothing happened, but in Thunderbird or Firefox redirects me to the bottom or the top email entry whether I am not writing a message or a post like here.
Regards,

Here is the output:

lepore@lepore-desktop:~$ inxi -c 5 -b
System: Host: lepore-desktop Kernel: 4.15.0-32-generic x86_64 bits: 64 Desktop: KDE Plasma 5.13.4
Distro: KDE neon User Edition 5.13
Machine: Device: desktop Mobo: Gigabyte model: GA-78LMT-USB3 R2 v: *** serial: N/A BIOS: Award v: F1 date: 11/08/2017
CPU: 8 core AMD FX-8350 Eight-Core (-MCP-) speed/max: 1406/4000 MHz
Graphics: Card: NVIDIA GP108
Display Server: x11 (X.Org 1.19.6 ) drivers: nvidia (unloaded: modesetting,fbdev,vesa,nouveau)
Resolution: 1920x1080@60.00hz
OpenGL: renderer: GeForce GT 1030/PCIe/SSE2 version: 4.6.0 NVIDIA 396.54
Network: Card: Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller driver: r8169
Drives: HDD Total Size: 5241.1GB (51.4% used)
Info: Processes: 272 Uptime: 5 days Memory: 4010.3/7830.3MB Client: Shell (bash) inxi: 2.3.56

To clarify a bit, shift+space does nothing in Chromium or Thunderbird in text entry fields, or the URL/email address fields. Just clarifying to differentiate between typing shift+space with nothing focused.