fix svgTextUtils for FF: use child element instead of SVG to getSize()

Author: antoinerg

src/lib/svg_text_utils.js

@@ -114,10 +114,11 @@ exports.convertToTspans = function(_context, gd, _callback) {
                 .style({overflow: 'visible', 'pointer-events': 'none'});
 
                 var fill = _context.node().style.fill || 'black';
-                newSvg.select('g').attr({fill: fill, stroke: fill});
+                var g = newSvg.select('g');
+                g.attr({fill: fill, stroke: fill});
 
-                var newSvgW = getSize(newSvg, 'width');
-                var newSvgH = getSize(newSvg, 'height');
+                var newSvgW = getSize(g, 'width');
+                var newSvgH = getSize(g, 'height');
                 var newX = +_context.attr('x') - newSvgW *
                     {start: 0, middle: 0.5, end: 1}[_context.attr('text-anchor') || 'start'];
                 // font baseline is about 1/4 fontSize below centerline