Truly transparent HDMI switch (2:1)?

[Jef06]

I need a two input, one output HDMI switch to connect two devices to a TV. I bought a random one which works with one device (a MacBook) but not with another one (a set-top box). I am a bit reluctant to buy another switch because I have no way to make sure it will work. It seems that none of these simple switches are truly passive, resulting in potential compatibility problems. Still it should be possible to just use high speed dumb multiplexers to implement a HDMI switch, no? (well except for the lines which need to carry some current but that should be easy to deal with). Anyone knows of an existing product that would be just that? Or maybe I was unlucky? All signals are 4K, nothing too fancy.
Thanks!