[Webkit-unassigned] [Bug 219204] New: Incorrect values of disabled (const) attribute when drawing with drawArraysInstanced

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=219204

            Bug ID: 219204
           Summary: Incorrect values of disabled (const) attribute when
                    drawing with drawArraysInstanced
           Product: WebKit
           Version: Safari 14
          Hardware: iPhone / iPad
                OS: iOS 13
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebGL
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: msokalski at novomatic-tech.com
                CC: dino at apple.com

Problem occurs on iOS 14.2, Safari 14, iPhone SE (2017)
It works perfectly on iMac(27" late 2013) Catalina 10.15.7, Safari 14.0

Using WebGL2 context, I have 3 attributes, 
2 are enabled and 1 is disabled in VAO.

#0 enabled  divisor = 1
#1 disabled divisor = 0
#2 enabled  divisor = 1

all other attributes are disabled and not used in shader program.


When drawing with drawArraysInstanced attribute #1 values, accessed in vertex shader, are invalid and vary. 
It should have a CONSTANT value set by ie: call to vertexAttrib4f().

Values of other two attributes (#0 and #2) are ok.

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