As I am writing this question, my Ubuntu 18.04 has again hit the problem that it often hits:

1. it is completely functional. I can open and close applications with a breeze through my keyboard.
2. I can move the mouse pointer around.
3. The mouse pointer icon is stuck, and clicking the mouse pointer anywhere does not do any interaction it at all. For example, clicking on the Chrome icon won't launch it, won't in fact do anything, however, launching it via the keyboard works. The same applies to any mouse clicks, they don't simply occur, as if nothing was clicked.

I checked the memory usage via top and it doesn't seem to be very high in total (<50%). What's the problem and how to fix it?

List of the gnome extensions I am using:

drwxrwxr-x 5 gt gt 4096 Dec 17 17:22  
drwxrwxr-x 7 gt gt 4096 Dec 17 17:25 
drwxrwxr-x 5 gt gt 4096 Dec 17 17:29 
drwxrwxr-x 2 gt gt 4096 Dec 17 17:29 

Output of free -h (not at the time my mouse pointer was stuck):

              total        used        free      shared  buff/cache   available
Mem:           3.3G        1.8G        255M         70M        1.3G        1.3G
Swap:          2.0G          0B        2.0G

sudo lshw -C memory:

*-firmware                
       description: BIOS
       vendor: Insyde
       physical id: 0
       version: F.10
       date: 07/25/2016
       size: 128KiB
       capacity: 8128KiB
       capabilities: pci upgrade shadowing cdboot bootselect socketedrom edd int13floppynec int13floppytoshiba int13floppy360 int13floppy1200 int13floppy720 int13floppy2880 int9keyboard int10video acpi usb uefi
  *-cache:0
       description: L1 cache
       physical id: 5
       slot: L1 Cache
       size: 256KiB
       capacity: 256KiB
       clock: 1GHz (1.0ns)
       capabilities: pipeline-burst internal write-back unified
       configuration: level=1
  *-cache:1
       description: L2 cache
       physical id: 6
       slot: L2 Cache
       size: 2MiB
       capacity: 2MiB
       clock: 1GHz (1.0ns)
       capabilities: pipeline-burst internal write-back unified
       configuration: level=2
  *-memory
       description: System Memory
       physical id: 1d
       slot: System board or motherboard
       size: 4GiB
     *-bank:0
          description: SODIMM DDR3 Synchronous Unbuffered (Unregistered) 1600 MHz (0.6 ns)
          product: HMT451S6DFR8A-PB
          vendor: Hynix
          physical id: 0
          serial: 065580E3
          slot: Bottom-Slot 1(Left)
          size: 4GiB
          width: 64 bits
          clock: 1600MHz (0.6ns)
     *-bank:1
          description: SODIMMProject-Id-Version: lshwReport-Msgid-Bugs-To: FULL NAME <EMAIL@ADDRESS>PO-Revision-Date: 2012-02-05 00:26+0000Last-Translator: Andi Chandler <Unknown>Language-Team: English (United Kingdom) <>MIME-Version: 1.0Content-Type: text/plain; charset=UTF-8Content-Transfer-Encoding: 8bitX-Launchpad-Export-Date: 2018-07-12 13:19+0000X-Generator: Launchpad (build 18719) [empty]
          product: Empty
          vendor: Empty
          physical id: 1
          serial: Empty
          slot: Bottom-Slot 2(Right)