Bug 295730 – REGRESSION (macOS 26): A large border-radius can cause shadows with a non-zero spread to render incorrectly

• Component: Layout and Rendering
• Status: RESOLVED DUPLICATE

[Jordan Pittman]

Created attachment 475866 [details]
An HTML file that showcases the buggy shadow behavior

In macOS 26 (I tested beta 3) Safari and Safari Tech Preview can incorrectly render box shadows when the border-radius is large enough and there is a non-zero spread value. For example, a spread of `1px` with a radius >= 16,781,246px draws the shadow incorrectly. Likewise, for a spread of `-1px`, a radius of `>= 16,781,379px` draws the shadow incorrectly.

The following HTML can be used to reproduce the issue. For the negative spreads you shouldn't be able to see anything and the positive spreads should produce a slight border around the circle. Instead a black square inset/outset by the spread value is drawn behind the element in all four cases.

```html
<div
  style="
    width: 8rem; height: 8rem;
    box-shadow: 0 0 0 -1px #000;
    border-radius: 16781379px;
  "
></div>

<div
  style="
    width: 8rem; height: 8rem;
    box-shadow: 0 0 0 -1px #000;
    border-radius: calc(infinity * 1px);
  "
></div>

<div
  style="
    width: 8rem; height: 8rem;
    box-shadow: 0 0 0 1px #000;
    border-radius: 16781246px;
  "
></div>

<div
  style="
    width: 8rem; height: 8rem;
    box-shadow: 0 0 0 1px #000;
    border-radius: calc(infinity * 1px);
  "
></div>
```

Notes:
1. I am testing in a VM so it's possible that the actual values are environment depdendent and might be slightly different.
2. Using `calc(infinity * 1px)` produces a much larger value and as such is also affected by this bug.
3. The other values of the box shadow do affect the rendering but it's still bugged — just with a different final appearance. e.g. Giving the shadow a radius causes a blur to appear but applied to an incorrectly sized "bounding box".
4. iOS 26 (I tested beta 3) is also affected.
5. Safari and STP on macOS Sequoia are **NOT** affected.

[Radar WebKit Bug Importer]

<rdar://problem/155561314>

[Simon Fraser (smfr)]

*** This bug has been marked as a duplicate of bug 295213 ***