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

[antoinerg]

Closes #2616

The fix was tested in Mozilla Firefox 62.0.3 in Linux. I try to fix the following misbehaving routine in FF:

plotly.js/src/lib/svg_text_utils.js

Lines 22 to 24 in fe842c4

	function getSize(_selection, _dimension) {
	return _selection.node().getBoundingClientRect()[_dimension];
	}

However, after inspecting the DOM in Firefox, it turns out that it sets the size of the embedded svg.gtitle-math elements to a very large value. However, it sizes properly its first and only child element. We use this one instead when calling getSize().

Codepen before: https://codepen.io/antoinerg/pen/oOEaPz
Codepen after: https://codepen.io/antoinerg/pen/ZZrqmd

Before:

After:

[etpinard]

Brilliant fix!

Wanna try to 🔒 this down in a test?

Good news: npm run test-jasmine -- --bundleTest=mathjax --FF appears to fail on master. (1) Wanna make sure it passed off your branch? Once (1) is done, simply adding one more task (corresponding to npm run test-jasmine -- --bundleTest=mathjax --FF in test_bundle.js should be good enough.

[antoinerg]

Good news: npm run test-jasmine -- --bundleTest=mathjax --FF appears to fail on master. (1) Wanna make sure it passed off your branch?

It works on branch fix-2616!

Once (1) is done, simply adding one more task (corresponding to npm run test-jasmine -- --bundleTest=mathjax --FF in test_bundle.js should be good enough.

I added a test in test_bundle.js for MathJax in Firefox in b1177a6. It gets executed on CircleCI as the last step:

[etpinard]

One of my favourite bug fixes of the year so far.

💃 💃 💃

[jonmmease]

Awesome!!! 🎉
This one has annoyed me ever since I got things working again in chrome last year. Thanks for digging in!

[etpinard]

To be released in tomorrow morning's v1.47.3