[Webkit-unassigned] [Bug 130124] New: [CSS Grid layout] Initial position in span not correctly computed sometimes

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=130124

           Summary: [CSS Grid layout] Initial position in span not
                    correctly computed sometimes
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: svillar at igalia.com


We're using std::lower_bound to find the first line before the opposite resolved position. The returned iterator might point to a resolved position which is actually greater than the opposite position (since lower_bound returns the first non less than). In those cases we need to move the iterator to the previous line so that the span can be correctly computed.

For example, let's imagine that we try to compute the span for a given named line which is defined in these tree positions (tracks)

[2, 5, 8]

Suppose that the opposite final position to the span was computed as 7. Then std::lower_bound will return an iterator pointing to 8. We're looking for the first position before 7, and obviously 8 is after, so we have to move the iterator to the previous position which would be 5. That's because how std::lower_bound works, if the iterator points to a value <= then we use it, but if it's > then we have to use the previous one.

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