Bug 83260

Hello!

I'm working on bug 83260 
(https://bugs.documentfoundation.org/show_bug.cgi?id=83260) and have an 
issue...
In short, when redlining (change tracking) is ON undo will crush LO 
during some circumstances.

This is connected with redlining implementation in LO. When you toggle 
Show in Edit->Changes->Show and doc already has some deleted paragraphs, 
these paragraph moved from special section of document do document 
section. As a result, such move invalidate Undo indexes and during Undo 
LO will crush in many cases.

For now I see 2 possible solutions:
1. Rewrite redlining, so Show option will change how doc is viewed, but 
not how it is formed (list of nodes)
2. When toggling change tracking or show option ON/OFF - clean 
Undo/Redo.

For now, I prefer option 2 :-)

Do we need to warn user, that we going to purge Undo/Redo (say, by 
displaying some warning?)
Any additional suggestions will be appreciated.