Bug 297620 – REGRESSION (Safari 18.x): backdrop-filter does not work when referencing CSS variable on macOS Sonoma

• Component: Layout and Rendering
• Status: NEW

[Jordan Pittman]

Created attachment 476457 [details]
Example file that showcases the bug

REGRESSION (Safari 18.x): Backdrop Blur does not work when using variable on macOS Sonoma

The following document does not apply a backdrop filter when using macOS Sonoma and Safari 18.6. Later OS versions work as expected:
```html
<style>
.blur-value-in-var {
  --a: 25px;
  -webkit-backdrop-filter: blur(var(--a));
  backdrop-filter: blur(var(--a));
}

.container {
  position: relative;
}

.overlay {
  position: absolute;
  inset: 1rem;
  width: 4rem;
  background-color: rgb(127 127 127 / 0.25);
}

.block {
  background: black;
  width: 4rem;
  height: 4rem;
}
</style>
<div class="container">
  <div class="overlay blur-value-in-var"></div>
  <div class="block"></div>
</div>
```

It seems to break whenever the filter uses a `var(…)` function. This can be the value portion or the entire filter function inside the variable. Additionally, I discovered that the unprefixed `backdrop-filter` property *does not work at all* Safari 18.6 on macOS Sonoma while it does in Sequoia and above.

I've attached a file with a couple more test cases.

Tested:
- macOS 14.6.1 + Safari 17.6 - works (except for unprefixed backdrop-filter obvs)

- macOS 14.6.1 + Safari 18.6 - broken
- macOS 14.7.7 + Safari 18.6 - broken
- macOS 14.7.7 + Safari TP - broken

- macOS 15.6 + Safari 18.6 - works
- macOS 26 b7 + Safari 26 - works
- macOS 26 b7 + Safari TP - works

Notes:
- I don't know the exact version of Safari this regressed in but I *think* its some version of 18 given that Safari 17.6 worked
- I wasn't able to test 298917@main on Sonoma w/ Safari 17.6 because it won't open any Safari windows.
- I wasn't able to test 298917@main on Sonoma w/ Safari 18.6 because it crashes.

[Radar WebKit Bug Importer]

<rdar://problem/158810759>

[Simon Fraser (smfr)]

Does this still reproduce in Safari in the final version of macOS Tahoe?

[Jordan Pittman]

No, this does not reproduce on Safari 26.1 + macOS 26. But it *does* reproduce when using Safari 26.1 on macOS 14.8.2 (at least in a virtual machine).