The latest 64-bit versions of Sigil dropped an internal function known as BookView. It was replaced by the option to call an external XHTML editor. The Sigil developers have made one specific for Sigil, this is PageEdit. Sigil can be set to call Page Edit as the external XHTML editor.
Since this is intended for the 64-bit version of Sigil it is 64-bit. No 32-bit vesion was released.
Not sure if you would add this to the Sigil installer package, make as separate app, or make it like a plugin. The user would have to configure Sigil to use it wherever it is placed.
"PageEdit is a single page visual XHTML editor based on Sigil's Deprecated BookView. It uses QtWebEngine instead of QtWebKit. It replaces much of what Sigil's BookView could do plus it integrates the QtWebEngine Inspector which allows you to see the result of your visual editing live, and make changes via the Inspector interface itself."
"PageEdit can be easily launched from Sigil (any recent version) with its Open-With functionality or set as Sigil's preferred external xhtml editor for fast launch by shortcut or icon.
Please remember that used as an External Editor for Sigil, requires that the user saves the file before exiting PageEdit and before further editing in CodeView to prevent loss of text/code."
PageEdit-0.9.5 Released - MobileRead Forums
Download (64-bit)Release PageEdit-0.9.5 · Sigil-Ebook/PageEdit · GitHub
Installation Windows PageEdit Stuff · Sigil-Ebook/PageEdit Wiki · GitHub
NOTE: Previously it was noted that PageEdit requires VC++ 2017 64-bit runtime unless Sigil 0.9.15 or later is installed.
PageEdit-0.7.1 Released - MobileRead Forums https://www.mobileread.com/forums/showthread.php?t=321649
The Windows installer for PageEdit is currently a self-extracting zipfile. Double-clicking it will cause it to extract its contents to a folder named PageEdit. Put the PageEdit folder anywhere you like. The PageEdit.exe file can be found in that folder. PageEdit requires that the VC++ 2017 64-bit runtime is installed (but does not include or install it itself). If Sigil-0.9.15 (or higher) is already installed, this will already be taken care of. Otherwise, you're free to download and install the 2017 runtime yourself if you don't use Sigil.