I just read in one of the other threads that the Editor's syntax highlighting was written from scratch. As such I wonder whether it might be possible to introduce a feature which I've found endlessly useful in Notepad++ over the years.
If one left-clicks anywhere inside a tag then that tag and its corresponding end-tag are highlighted. e.g. in the following html:
<p class="tx"><span class="bold"><span class="it"><span class="dropcap">T</span><span class = "sc">his</span> is a paragraph</span> with lots of spans.</span></p>
Clicking anywhere in the <span class = "it"...> tag would show like this (
except NotePad++ colours background rather than foreground text):
<p class="tx"><span class="bold">
<span class="it" id="urhere"><span class="dropcap">T</span><span class = "sc">his</span> is a paragraph
</span> with lots of spans.</span></p>
The lack of this feature in Sigil often made sorting out spaghetti nested html easier in the old ebook Tweak for me because I could use my normal editor. Retail Publishers seem to love endless nested divs which don't actually
do anything.