[Webkit-unassigned] [Bug 258014] New: Navigation Timing L2 often returns negative timing values for documents served from memory cache

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=258014

            Bug ID: 258014
           Summary: Navigation Timing L2 often returns negative timing
                    values for documents served from memory cache
           Product: WebKit
           Version: Safari 16
          Hardware: iPhone / iPad
                OS: iOS 16
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Page Loading
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: ad at andydavies.net
                CC: beidson at apple.com

Created attachment 466677

  --> https://bugs.webkit.org/attachment.cgi?id=466677&action=review

Screenshot showing negative NT2 values

Description:

Navigation Level 2 API returns negative values for several metrics when the document is served from memory cache.

Behaviour doesn't happen 100% of the time but seems common enough to make it easy to reproduce.

Behaviours hasn't been reproduced when document comes from the network, or the disk cache.

When there are negative values in NT2, the values calculated via NT1 appear to be reasonable

Negative values appear to be constrained to: 


Reproduction:

- Open speedcurve.com
- Navigate between a couple of pages until the Inspector shows the document has been loaded from memory cache
- Query the NT2 API i.e. performance.getEntriesByType('navigation')[0]; 
- Observe negative values (steps 2 & 3 might need repeating a few times)


Notes:

Tested on iPhone with iOS 16.5, Safari 16.5 (17615.2.9.11.6, 17615) and STP 171 (Safari 16.4, WebKit 17616.1.15) running on Intel Mac

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