In very simple cases of feOffset used on its own such as https://developer.mozilla.org/en-US/docs/Web/SVG/Element/feOffset#example , the subregion fed to WebRender isn't being calculated correctly in Gecko (BuildWebRenderSVGFiltersImpl), this is blocking enabling feOffset which is a semi-common filter.
Bug 1902136 Comment 0 Edit History
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If gfx.webrender.svg-filter-effects.feoffset is enabled (which is not presently the default) the bounds of the second rect in https://developer.mozilla.org/en-US/docs/Web/SVG/Element/feOffset#example turns out wrong (appearing as if pre/post filter bounds is being used incorrectly), presumably the subregion fed to WebRender isn't being calculated correctly in Gecko (BuildWebRenderSVGFiltersImpl), this blocks enabling feOffset which is a semi-common filter.