3 days ago I bought a laptop, new, with an R9 M385X card.
On initial launch of SL (Firestorm - most recent build), and after enabling Advanced Lighting, I had the weird pixelated line, exactly the same as in the JIRA.
I did notice that the line DID NOT render in snapshots, which was odd, but made me less angry, as I do a lot of photography.
The next day, I launched SL (Firestorm) again and received a message -
"Your graphics settings have been changed, Firestorm has recognized your graphics device has been changed from ATI Radeon R7 to ATI Radeon R9"
I turned on advanced lighting - no line.
I turned on shadows - no line.
No line anymore on any setting..what?
So - why did this happen?
The only thing I've changed between the two instances of SL, was an issue I was having with Windows 10 background updater (BITS) - it was stopped, and I got it running again.
This may have nothing to do with it, but it may be possible a driver updated? Did AMD address the issue? I have no idea.
But, I did want to share my experience with this issue.
I hope maybe this information helps.