Update: I found if I delete the scg wrapper div and set the width and height on the svg element to 100% it works as required. It DOES NOT work id the width and height are set in the CSS which seems wrong as they are suoposed to be equivalent as far as I can tell. This all seems a bit hacky as I'd expect the flow box' basically "outside in" layout to set the svg viewport correctly.
Bug 1592997 Comment 1 Edit History
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Update: I found if the same problem in the eqivalent *grid* layout but in that case if I delete the svg wrapper div and set the width and height on the svg element to 100% it works as required. It DOES NOT work if the width and height are set in the CSS which seems wrong as they are suoposed to be equivalent as far as I can tell. This all seems a bit hacky as I'd expect the flow box' basically "outside in" layout to set the svg viewport correctly.