This comment is in reference to the following closed thread:https://forums.elderscrollsonline.com/en/discussion/599446/cant-steer-with-mouse-any-longer
I had this same issue after I applied the latest ESO patch on my MacBook Air this past Tuesday morning (May 3). It's happened before, and I'm not sure what causes it, but I don't think it has anything to do with the internet or network connection. When it happens, the only way I can steer with the mouse is to hold down the right mouse button while moving the mouse, even though the mouse appears to function as expected outside of the game.
When it happened Tuesday morning, I checked the mouse bindings in the game settings and everything looked correct, so it doesn't seem to be anything to do with the game settings being changed by applying a patch.
In my case, the solution has been to turn off my wireless mouse, turn it back on, and go into the OS settings to reselect the wireless mouse and reconnect to it. After doing that, the Mouse Freelook (or "steering with the mouse") works perfectly again.
I mention this in case it's helpful to anyone else who might run into this issue.
I've fought mudcrabs more fearsome than me!