When I am in block mode and moving the screen about, to read the code of my various modules, using the left mouse button, if I accidentally bump the right button at the same time my modules scatter all over the screen!?
This might not be a bug but it sure does bug me 
OK yes my mouse has a light touch ! 
Good thing I keep many many backups 
Why does this happen and is there a fix or a way to avoid it?
Buy a better mouse? 
Let me introduce my below post, you can easily find your procedure /event WITHOUT move around on the block area by mouse.
Trackballs with scroll wheels provide room to stay away from the wrong button.
True but ...
What is it doing when I hit the right button?
You might need to hold that right button down until you leave the right click selection on the most harmless option in that right click menu.
I did not find any menu for "scatter all over the screen",
do you mean these ?
I suppose it must be that its rearranging in a most horrible way. The blocks didn't seem to particularly arrange either way .... very confusing.
Considering I have maybe 70 modules on each screen it means rearranging everything is more than an hour of work. Yes modules ... over 2,000 blocks on one screen. I have 8 screens. two of the others have about 1,400 blockes and the others only maybe 800-1,000
It does cause me to wonder if the inventor was designed for monster apps
Seriously - why isn't there a lock screen option?
EDIT: just after writing this reply I did it again!!! I had to go through the delete and then delete trash then retrieve the aia from backup. To reassemble is so painfull.
I then created a lock screen by putting a rubber wedge under the tiny slit of the right mouse lip 
Have you tried right click Undo when this happens?
thanks so much TIMAI2. I never noticed it !!!
New mouse and I simply freaked out. Nothing like missing the obvious right?
I avoid any of this by using a right handed mouse with my left hand (being left handed), which means my index finger is on the left button, no finger hovering over the right button.
(I chose this method a long time ago, when providing support to computer users, most of whom were right handed, made it easier to use their mouse 
)
Try a "handshake" mouse - more ergonomic.
Google "handshake mouse"
Lucky for you being south Paul ey?
Its super scary to see 200+ blocks go scrambling all over the scree! It was a huge drag on MIT, the screen locked up. I imagine it might take 5 minutes or more to finish and "undo" would take the same amount of time of course. Therefore "Undo" wouldnt be very usefull to me. In my case I must make backup with each revision no matter how small, disconnect'close MIT AI, Open MIT AI, delete and then delete trash and then import the backup aia. No matter how you slice it the process is long and freightening, especially if you made large or complex changes.
Again I would like to emphasize that there should be an alternative way to avoid this
Thanks for that ChrisWard ... I will look into that
"scatter all over the screen"
Next time if this happen again, just close your browsers quickly, then re enter your project.
Thus there is no time for app inventor have time to store the status.
Thanks but I tried that and it did store 
I think I know what is happening, when he accidentally clicks the right button and swipes the screen holding up that right button, the website changes the page (goes back to the previous page if swiped left, and moves forward to the next page if swiped right).
This is a browser gesture, I just tried it with my Opera browser.
NO that is not what is happening. No change of pages.
Right-Click on your project page ... you will see this:
It is simply auto selecting Arrange Blocks
That may work in a web browser but in MIT AI all it will do is (left) move the screen or (right) pop the above menu
Don't think we have asked, are you developing on a computer, or ipad/tablet ?
PC TIM. not laptop or other
I know this is a popup menu but this has me thinking that maybe some of the items should have a confirmation after selecting?
There have been many reports of problems with the pop-up menu, but Mit has no intention of fixing it. Now all heads are working on the iOS version. The solution would be to disable the ability to select options from the right-click menu. In my opinion, it should be that we open the menu with the right button, and we select functions from the menu only with the left button. Currently, functions from the menu can be selected with both the left and right button. This wouldn't be a problem if in certain situations the menu did not appear under the mouse pointer, but only next to it.