Created attachment 67669 [details]
Screenshot showing the problem

When using a gtk3 theme that uses "border-image" with the "stretch" as repeat mode (such as Adwaita, the default theme), gtk3 uses cairo to scale the image. When using the "image" backend of cairo everything looks as expected. Using the X backend on a "intel" driver should work as well (as I'm told). But using the "nouveau" driver gives some fading out borders.

After changing NV50EXAPictTexture to use CLAMP_TO_EDGE for RepeatPad like the intel driver does (http://cgit.freedesktop.org/xorg/driver/xf86-video-intel/tree/src/i915_render.c?id=c6008068372709c73034163eddc902b47bf87d24#n327) the result looks like the one rendered with the "image" backend.

I'm attaching the patch and a screenshot that shows the rendering issue, how it looks with the image backend and after the patch.

For the screenshot I've made the top right pixel in the top border red (#ff0000). In the first screenshot you can see how the color "fades away" because it gets blended with some transparent pixels when scaled. In the second and third screenshot the rightmost pixel before the corner is #ff0000.

I've only tested the patch for nv50 but the nv40 one should work as well if the defines are correct.