Comment 3 for bug 1652770

The original problem which why I uploaded the file was the "red diagonal" not going through the corner.

After the upload realised that the thumbnail showed a square with a solid fill, generated with 0.91.
That issue is unrelated to the reported problem.

So to clarify it a bit more.
The filtered object is a square with a black to white linear gradient fill going from top left corner to bottom right, meaning that the diagonal from bottom left to top right should be filled with a perfect midtone, half between absolute dark and light.
The filter applied uses an fecolormatrix in luminance to alpha mode, and the component transfer is added to the alpha channel of that.
Upon the component transferring, flood fills are used and compositing to fill up the areas with the same alpha values, resulting in a red and a transparent half
-red should indicate areas where the source's luminance is less than the midtone's, and transparent where it is lighter than that.
By the discrete input set to 0 1, it is supposed to make that "cut" right at the middle, but somehow it is shifted at the rendering. Same goes with the linear input set.

The gradient bending leaves a step right through the corner so it's probably not about the low colour depth rendering of the srgb.