The attached test case shows that the rendering of the mo element is not updated when its contents change.
Created attachment 87965 [details]
The patch for bug 52444 also fixes this bug.
(In reply to comment #2)
> The patch for bug 52444 also fixes this bug.
That was the old giant January patch for 52444, not the smaller one finally landed in May. I'll include the fix in a new patch as I continue going through the MathML code.
This seems to be fixed.
Closing the bug since the test case passes.
There are some dynamic tests in LayoutTests/mathml/presentation/tokenElements-dynamic.html.