Box shadows with blur and spread increase the corner radius too much

[mstange]

https://bugzilla.mozilla.org/show_bug.cgi?id=1429793

Testcase

This shadow has a spread radius.

When the shadow has no blur, then the shadow's corner radius is correct. But if it does have a blur, then the corner radius is too large.

Expected vs actual results:

---
root:
  items:
    - type: stacking-context
      bounds: [0, 0, 1000, 1000]
      items:
        - type: box-shadow
          bounds: [ 0, 0, 200, 200 ]
          color: blue
          clip-mode: outset
          offset: 250 50
          blur-radius: 1
          spread-radius: 40
          border-radius:
            top-left: [0, 0]
            top-right: [120, 120]
            bottom-left: [120, 120]
            bottom-right: [0, 0]
        - type: box-shadow
          bounds: [ 0, 0, 200, 200 ]
          color: blue
          clip-mode: outset
          offset: 50 250
          blur-radius: 0
          spread-radius: 40
          border-radius:
            top-left: [0, 0]
            top-right: [120, 120]
            bottom-left: [120, 120]
            bottom-right: [0, 0]

[mstange]

I discovered this on the box-shadow playground.

[nical]

What seems to be happening is that per spec we are adjusting the border radii of the rounded rect we are using for the shadow, but with a wrong spread value.
If I divide the spread value we use in adjust_radius_for_box_shadow by two, the result looks correct.
Or the radii are adjusted twice maybe.

[mstange]

That sounds like we're doing the adjustment twice instead of just once.

[nical]

Looks like it's a tad more complicated than my hunch above. One thing that strikes me as odd is that the rounded rect that we send to the shader has the correct adjusted border radii, but unadjusted rectangle dimensions. Yet the final blurred rectangle gets drawn at the correct size later and I haven't seen where the dimensions are adjusted. In any case the rectangle that is blurred out has a mismatch between the actual size and the radii and I am pretty sure the problem is there.
Edit: I kinda see how that could be made to work with the nine-patch path, but in the case I am looking at we are not using nine-patches.