Bug 300232 – Incorrect safe area values with scrollview inset and obscured inset (in clients other than Safari)

• Component: DOM
• Status: RESOLVED FIXED

Created attachment 476976 [details]
Example project

When both the scrollview inset and the obscured inset are set, the reported safe area values are incorrect.

Test website: https://adangla.github.io/check_css.html (source: https://github.com/adangla/adangla.github.io/blob/main/check_css.html)
You can find an example app in the attachment (safe-area-obscured-inset-ios26.zip).

Expected behavior: 
CSS variable returns 0 when the top toolbar is larger than the safe area.

Actual Behavior:
Instead of returning 0, the safe area CSS value (or equivalent) returns 62, which is the standard top safe area value. This indicates that the system is not correctly accounting for the combined occlusion from both the scrollview inset and the obscured inset, leading to an inaccurate representation of the truly safe usable space.

Created attachment 476977 [details]
Example on Safari

Note that in this case the behavior is correct, the top safe area is equal to 0.

Created attachment 476978 [details]
Example of broken behavior

Note that in this base the top value is not 0 (like in Safari) but it's 62.
It can be reproduced from the attached test project.

Note that we were not able to reproduce this on Safari because the top toolbar is bigger than the safe area.
We can only reproduce this issue when this is not true.

Please let us know if you need additional information.

[Radar WebKit Bug Importer]

<rdar://problem/162521096>

[Wenson Hsieh]

Thanks for the report! I can reproduce on 26.1.

Debugging into this a bit, it seems this only works in Safari because Safari uses `-[WKWebView _setUnobscuredSafeAreaInsets:]` to override unobscured safe area insets, which sets the `_haveSetUnobscuredSafeAreaInsets` flag; the value is then returned here:

```
- (UIEdgeInsets)_computedUnobscuredSafeAreaInset
{
    if (_haveSetUnobscuredSafeAreaInsets)
        return _unobscuredSafeAreaInsets; // < - - - -

    if (!self._safeAreaShouldAffectObscuredInsets) {
        auto safeAreaInsets = self.safeAreaInsets;
#if PLATFORM(WATCHOS)
        safeAreaInsets = UIEdgeInsetsAdd(safeAreaInsets, self._contentInsetsFromSystemMinimumLayoutMargins, self._effectiveObscuredInsetEdgesAffectedBySafeArea);
#endif
        return safeAreaInsets;
    }

    return UIEdgeInsetsZero;
}
```

We probably need to adjust the code in the `!self._safeAreaShouldAffectObscuredInsets` case below the `_haveSetUnobscuredSafeAreaInsets` early return…

[Wenson Hsieh]

Pull request: https://github.com/WebKit/WebKit/pull/52362

[EWS]

Committed 301534@main (af80de949632): <https://commits.webkit.org/301534@main>

Reviewed commits have been landed. Closing PR #52362 and removing active labels.