Fix changing text formatting style does not work correctly when selecting multiple

[dab246]

Issue

Changing text formatting style does not work correctly when selecting multiple

Root cause

Because document.onselectionchange is only fired when the selection changes, not when the content or formatting of the selection changes. When document.execCommand('italic') applies the formatting but does not change the selection, so onselectionchange is not called again.

Solution

Use MutationObserver to listen for DOM changes.