the caller javascript should wait (if it is to work properly), so I guess https://searchfox.org/mozilla-central/rev/0671407b7b9e3ec1ba96676758b33316f26887a4/dom/svg/SVGFEImageElement.cpp#254 should simply return true rather than false.
Bug 1559715 Comment 18 Edit History
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the caller javascript should wait (if it is to work properly), so I guess https://searchfox.org/mozilla-central/rev/0671407b7b9e3ec1ba96676758b33316f26887a4/dom/svg/SVGFEImageElement.cpp#254 should simply return true if the load is not complete rather than false if it is.