I'm not sure if this is a bug or just something my system is doing so any help/guidance would be appreciated.
I noticed this yesterday that when entering text into 'boxes" in Firefox I could not select an option from the drop down list presented. Seemed to always select the top entry. Not immediately important - will look at tomorrow i thought.
Today when i try and move a window I grab hold of the title bar and start to move - less than a second later the window stops moving (as I appear to have let go of the title bar - i didn't).
Also today - went into tweak tool to see if changing from Redmond theme to something else (and then back again) would fix the issue. Just putting the cursor over the layout drop-down (not clicking) selects the traditional theme - weirdness. (I did briefly go back to traditional as a test no joy)
So I selected Contemporary Theme, but now putting the cursor over the layout drop-down (not clicking) selects Cupertino.
Is anyone else experiencing this issue? - i suspect not (or perhaps I don't know what to search for as I'm a bit of a noob and might be searching for the wrong things)
Suggestions on what to do to fix the issue? - What other things do I need to tell you to help diagnose?
Been really happy with mate for some time now - but the frustration level is rising - PLEASE help..
I tried ‘the other’ operating system on my machine and the problem was NOT evident.
Problem was also NOT evident on a live CD (Lubuntu) image I had
I’ve decided to do a clean install (nuke and pave) - rolling back to 18.04 - and see if the issue comes back when I gradually reinstall the software I use.
Tried a live CD version of both Mate 18.04.01 and 18.10 and BOTH were exhibiting the same behaviour. These are live images off the website with no updates applied.
I’m about to go and download Kubuntu and see how it behaves (and perhaps a few others). Will let you know
Not 100% sure if it’s relevant, but one thing you can try is installing the old touchpad driver (synaptics). Ubuntu MATE 18.04 and 18.10 default to the newer libinput, which is generally better, but not always for all hardware, especially old ones. (Which is installed by default varies by Ubuntu flavour: Ubuntu desktop 18.04 and 18.10 also use lib input, but I think Kubuntu and Xubuntu still use synaptics by default.)
sudo apt install xserver-xorg-input-synaptics
and reboot to use it.
Edit: I guess you can’t reboot the live session to use it I’m not sure if just logging out and back in would switch that driver. If not, then you could try Xubuntu or Kubuntu which I believe still use the synaptics driver.
I finally decided to climb under the desk and pull the machine out of the corner (it’s not as easy as it sounds due to bookcase that was blocking moving a filing cabinet that was blocking moving the desk that meant I could lift the machine ‘up’…).
Upon doing this I discovered a long forgotten wireless mouse (I think) dongle plugged in. Removing it solved my issue.
Must have been there for years (the dust certainly would suggest ‘not a recent addition’ :-)) but oddly only started showing the new behavior recently… [ something new in the house/neighbors that is on the same frequency??}
Anyways - sorted now. THANKS for all the suggestions
Worn out or dirty mice/touchpads can start behaving erratically. 
I’m not sure if just logging out and back in would switch that driver. If not, then you could try Xubuntu or Kubuntu which I believe still use the synaptics driver.