Do you have failure reports in your system info? If you reply with the IDs we can look at the crashes.
I have the same issue, also with I3, but it's not a crash, I just can't click on any of the buttons in these event windows. Same with Steam update windows.
I also have this problem using bspwm.
It happens with event notifications and update notifications.
I do not have this problem friend list or chat windows.
I would guess this is the problem:
WM_HINTS(WM_HINTS):
Client accepts input or input focus: False
Nothing extracted yet.
I use i3 on Ubuntu, and popup windows that steam creates for things like event notifications always appear as tiled windows on my currently active workspace. Clicking "Close" or "View Event" on the popup does nothing, nor can I kill them via the window manager (Alt+Shift+Q). Occasionally clicking close will, instead of doing nothing, cause steam itself (along with the event notification) to close.
My current solution is to just stick all of these notification windows in the same workspace as steam just to get them out of the way.
Would be nice if these windows were killable/didn't crash. I would think that if you created the windows with a hint to the wm to float them (google chrome, eg, does this with webpage alerts) that the other issues might go away (in general they should probably be floated windows anyway)