black border with MBPro and 10.10.2 on external 3840x2160 TV (worked with 10.10.1)

Up to 10.10.1 have been successfully using external 3840x2160 TVs and monitors (Seiki and Philips) with my MacBook Pro (Retina, 13-inch, Late 2013) and Iris graphics.

After the update to 10.10.2, when connected to my Seiki 4k TV the MBpro now produces a black border around the active area (2cm left and right, 1cm top and bottom). Before the update, 10.10.1 used the entire width and height of the screen (and so do my other FreeBSD and chromebooks in 3840x2160).

With screen capture (Shift-Command-4) the cursor on the screen shows coordinates 0,0 at top left and 3839,2159 at the bottom right, which makes me think that the graphics card is generating incorrect timings.Fonts, as you can imagine, are a disaster in this interpolated mode.

I would really like to have this regression fixed.