The setting to use single or double clicks for navigation in Thunar seems to apply to middle-clicking as well as left-clicking. 

I think this shouldn't be the case, as middle-click serves no other purpose when using it on a folder, and it's odd to have to double-middle-click (I've never encountered this in any other applications.)

Of course, you can enable the single-click navigation behaviour, but then you have to deal with left clicking having this behaviour too, when they should be separate.

I'm having this issue with Thunar 1.6.4, which I didn't specify in the form with this report as that option was missing from the form (latest there is 1.6.3)