Measure repositioned tex in its own units

repositionMathGroup now measures the typeset group with getBBox, which
reports its size in the group's own units, instead of its client rect. The
size no longer changes when the group or the graph is rotated or scaled,
and it matches what positionText uses for plain text.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Author: SergeyAxiomatic

src/lib/svg_text_utils.js

@@ -65,6 +65,18 @@ function measureMath(newSvg, width0, height0) {
     return {width: w, height: h};
 }
 
+/**
+ * The size of a typeset group in its own units, which unlike its client rect
+ * does not change when the group, or something above it, is rotated or scaled.
+ *
+ * @param {d3 selection} mathjaxGroup: the '<svgClass>-group' wrapper
+ * @return {{width: number, height: number}}
+ */
+function getLocalSize(mathjaxGroup) {
+    var bb = mathjaxGroup.node().getBBox();
+    return {width: bb.width, height: bb.height};
+}
+
 /**
  * Positions a MathJax-typeset group against its source <text> node's x/y
  * and text-anchor.
@@ -260,9 +272,8 @@ exports.convertToTspans = function(_context, gd, _callback) {
  * convertToTspans, to match its source <text> node's *current* x/y and
  * text-anchor.
  *
- * The size is measured from the page, so it changes with the rotation or
- * scale of whatever the group sits in. A caller that moves the text into such
- * a state should measure once beforehand and pass that size back in.
+ * The size is measured in the group's own units, so it does not change with
+ * the rotation or scale of whatever the group sits in.
  *
  * @param {d3 selection} _context: the source <text> element
  * @param {{width: number, height: number}} [size]: a size returned by an
@@ -279,7 +290,7 @@ exports.repositionMathGroup = function(_context, size) {
     var newSvg = mathjaxGroup.select('svg.' + svgClass);
     if(mathjaxGroup.empty() || newSvg.empty()) return null;
 
-    size = size || measureMath(newSvg);
+    size = size || getLocalSize(mathjaxGroup);
     var textAnchor = _context.attr('text-anchor');
     var fontSize = parseInt(_context.node().style.fontSize, 10);
     positionMathGroup(_context, svgClass, mathjaxGroup, newSvg, textAnchor, fontSize, size);