aspect menu in Game Window distorts entire Canvas in Scene Window

So, I created a Screen-Space-Overlay Canvas and built my "Log into game" UI. Then I previewed it in the Game Window and everything looked great. Then I noticed that I had "Free Aspect" selected, so I changed it to "16:9" because that is my target aspect. Then the whole canvas bugged out. Here is a video of what happens:

https://www.screencast.com/t/hTgsm47TpM

How can the Game Preview Window affect the Scene View?? This baffles me.

Also, my canvas is set to Scale With Screen Size, with a Reference Resolution of 1366x768, which IS 16:9 aspect, so how does this damage the sizes of all of the elements on the canvas?