I usually work in non-standard frame size for my FCP projects because my final output will be in a 2:35:1 ratio. I use a Blackmagic Ultrastudio Mini Monitor as an HDMI output to a client monitor. Up until a few months ago the AV output of FCPX would allow the Mini Monitor to accept a frame size of 1920x818 and letterbox the output to 16x9 on this consumer grade LCD. One day, this just stopped working... I've reached out to Blackmagic and they have no solution or reason why this happened, and are not willing to offer support since what I was achieving wasn't an "officially supported" feature. Now, the AV output will distort the image vertically to fill a 16x9 frame. At this point I am stuck having to use either a matte layer or a widescreen effect on individual clips to output correctly to my monitor which increases edit time having to re-render any edits underneath the matte and having to re-order the letterbox effect after apply color correction etc. It was also very easy to adjust the frame offset with this wide angle project size.
Is anyone editing in 2:35:1 (or other cinema standard) projects and getting successful conversion on output? What are you using?