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DiapDealer, Trane - I was getting something similar last year, Sigil misbehaving when adding video or audio. I was exploring the possibility of using EPUB3 for a particular project.
The problem went away when I finally disabled
all my shell extensions. It was part of product that I've used for decades. I was averse to ditching it because in all that time it has never effected any other programs, and it's part of a package that provides me with tools I use all day, every day, such a global snippets manager, a clipboard manager etc.
The software is no longer available, and I have no idea where to find a usable copy of the installer. It was payware and its license key was restricted to a MAC address.
The product name is ShellToys, its producer was Cool Focus, they were based in the UK (Essex I think), they went broke as a result of the 2008 crash. Apparently they made an unrestricted version of the installer available to existing licensees, but I didn't get the email, because they had my old Lavabit address, and we know what happened to them...
I have tried to find a copy of the unrestricted installer - no luck. There was a free version called Shelltoys XP, it may be available somewhere, can't recall the differences between the it and the payware version.
Ah-ha... I found the thread, this is the last post ==>>
https://www.mobileread.com/forums/sh...3&postcount=42
Interestingly the problems I saw in 0.9.5 didn't happen in Sigil 0.8.4.
But we didn't use EPUB3 for that project, instead we created some apps using the Unity x-platform development tool.
BRrt