The first two matrices in comment 12 are actually inverses of each other, however the 883.99981689453125 in the first matrix can't be represented accurately enough by a float so that the -1024 doesn't show up in the product. It should be 883.999893529, note how it starts differing after 7 digits, ie float precision. So that might lead to a solution: if the transform matrix of an apzc times it's inverse is too far from the identity then we don't allow it to produce a hit. I looked at why layers isn't affected and I think it's just dumb luck: when testing if the apzc for the body element is a hit we determine that it is not (which is incorrect) and so we hit the html element and we avoid any big scales.
Bug 1737722 Comment 15 Edit History
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The first two matrices in comment 12 are actually inverses of each other, however the 883.99981689453125 in the first matrix can't be represented accurately enough by a float so that the -1024 doesn't show up in the product. It should be 883.999893529, note how it starts differing after 7 digits, ie float precision. So that might lead to a solution: if the transform matrix of an apzc times it's inverse is too far from the identity then we don't allow it to produce a hit. I looked at why layers isn't affected and I think it's just dumb luck: when testing if the apzc for the body element is a hit we determine that it is not (which is incorrect) and so we hit the html element and we avoid any big scales. Attached the hit testing tree and hittest comparing layers and webrender.