Same behaviour mentioned in bug 16820 and bug 17933: Composition with 24-bit source pixmaps does not result in alpha=0 outside source surface bounds, instead it samples as black a=255, rgb=0. Tested on AMD Mullins running an updated instance of Fedora-23 (Xorg 1.18.1, radeon 7.6.99, Linux 4.4.4).
What's your test case?
Ideally, there'd be a rendercheck test case exercising this. Meanwhile, this is more likely an issue in glamor or the Mesa radeonsi driver than in the Xorg driver. Reassigning to glamor for now. (Maybe you can try if it also happens with the modesetting driver using glamor on other hardware?)
Created attachment 122306 [details] [review] glamor: Only force alpha to 1.0 inside source pictures Does this xserver patch fix the probem?
Created attachment 123778 [details] self-contained testcase Thanks a lot for looking into this issue. Unfortunatly I can't compile X on the machine where I saw this issue, so I decided to create a self-contained testcase.
Comment on attachment 122306 [details] [review] glamor: Only force alpha to 1.0 inside source pictures https://patchwork.freedesktop.org/patch/88805/ fixes the attached test case for me.
commit 4711ebc174206b5a50e6ae8a7f974cd835e4ebd3 Author: Michel Dänzer <michel.daenzer@amd.com> Date: Tue May 24 18:12:42 2016 +0900 glamor: Fix sampling outside of RGBx source/mask pictures
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