When editing a complex formula, it is easy to get lost in a sea of MathJax code. The suggested feature is, when editing posts with MathJax:
(1) When a portion of a MathJax code is selected in the edit box, the corresponding portion of its result in the preview is highlighted.
(2) If none of a MathJax code is selected in the edit box, but the cursor in the edit box is inside a MathJax code, then one character at the corresponding position of the MathJax result in the preview is highlighted.
(3) When a portion of a MathJax result is selected in the preview, the corresponding portion of its code is highlighted in the edit box.
(4) When a MathJax result is clicked in the preview, the cursor in the edit box is set to the corresponding code of the clicked position in the result.