Webrender: Bottom left border radius isn't drawn correctly (on macOS 10.12)

[qezz]

Hi, I'm trying to use webrender as a window renderer.

So, here is an example I use to render a square window with the transparent rounded borders (corners):

https://github.com/qezz/webrender-bottom-left-border-issue/blob/master/src/main.rs

It appears to happen on mac only.

Expected output:

• all borders are rounded

Actual output:

• Bottom left border isn't rounded

(The black background is a terminal window provided just for the demonstration. Top-left, top-right, and bottom-right borders are rounded and the corners are transparent)

Configuration

• MacBook Pro (Retina, 13-inch, Early 2015)
• macOS 10.12.6 (Sierra)
• Intel Iris Graphics 6100 1536 MB
• webrender std output:

OpenGL version 4.1 INTEL-10.25.19
Shader resource path: None
Device pixel ratio: 2

• EDIT2: webrender version: rev = "34a498f7e46c385a189299e7369e204e1cb2060c"

EDIT: Probably related issues:

• #2754
• https://github.com/servo/servo/issues/20922

[kvark]

I'm able to repro (on Mac only), looking into it...