True WYSIWYG LaTeX System
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Centaur + TeXWord
using TeXWord as TeX Preview.

You can open the same file simultaneously in both programs: TeXWord and Centaur. File changes are automatically synchronized between these programs. The synchronization is implemented via immediate saving of a file to the disk whenever the program loses the focus, and loading it back when the program receives the focus again. When TeXWord reloads the changed file, it applies incremental formatting, instead of full processing of the document. So, that TeXWord may be used as the fastest TeX preview tool when you edit documents in Centaur. This approach may be useful for editing pages with drawings too complex for instant editing in TeXWord due to slower response.
Instant Preview (Windows Only)
Beginning from BaKoMa TeX 10.65 TeXWord running in background
can instantly preview changes made by another program (i.e. Centaur).

It is alternative solution of Feature Request: Multi-Threading - Text Edit + Text Processing

This feature is controlled in the dialog 'Options/Editing Settings/AutoSave'
by check box 'Reload files when changed by another programs' (default is off).

Centaur should be configured (in similar dialog) for more instant autosave (set 0 for idle save).

So, we can edit document in Centaur and see changes in TeXWord almost instantly.
Long files may increase the delay. However, document split into several files/chapters will have less delays,
because main overhead is related with saving and reloading file.

There is video demonstrating of instant preview of document edited on page 106: