I have Multiplicity 3.57 (Windows). In seamless mode, the program is great for me except for one annoyance. I very often could accidentally send my keystrokes to the secondary PC, believing they are being sent to the primary. This usually happens because the mouse was over the boundary to the second PC, whether it was accidentally nudged, or forgotten that this is where it was last positioned. It would be great if Multiplicity could be a bit more intuitive in this area. I think that if Multiplicity could pay attention to window focus something could be done with this.
Example solution use case: 2 PCs, PCPRIMARY and PCSECONDARY. I have Chrome in focus on PCPRIMARY. I move my mouse over to PCSECONDARY. Chrome in PCPRIMARY still receives my keyboard input until I interact with PCSECONDARY in such a way that it puts a window (or the desktop) into focus. Only at this point does keyboard input begin to be redirected to PCSECONDARY. Also, at this point, Chrome on PCPRIMARY is unfocused, so that when the user brings their mouse back to click on it, new focus can be detected and keyboard input returned back to PCPRIMARY.
Possible caveat: In case unfocus just changes focus to other windows (i.e. must have something in focus at all times - iirc this might be the case) - perhaps put focus on a dummy off-screen special window that Multiplicity can hold focus.
I think that such a solution would make seamless mode feel so much more seamless - and less prone to situations such as accidentally typing things into a chat window, clobbering text in a word document, accidentally toggling a button via spacebar, etc. I have 6 monitors - paying attention to where the mouse is at every moment isn't on the table - if I see a blinking cursor, the mindset is that this is where my text is going to go.