Sumatra PDF is a PDF, ePub, MOBI, CHM, XPS, DjVu, CBZ, CBR reader for Windows

Contents/Bookmarks panel missing for some CHM files

I use SumatraPDF for viewing .chm-files and for the most part it works fine.
But with this file:
the left sidepanel (topic panel?) is not shown.

I MS’ helpView, it looks like this:

Also looks fine in KchmViewer (but that program is very slow). I prefer SumatraPDF.

The VS6.CHM is pretty old (from 2002). Maybe that is related?

It is a special 2part Hybrid file (in some ways similar to a PDF portfolio with nested embedment’s) it requires the separate CHI Compiled Help Index

For such cases there is the option to open as I did here in Microsoft Help
but without the chi file there is no index for HHelp to use either.
When the CHI is added then MS can use it but SumatraPDF can not use it in either of its own Help file modes. Other dedicated Help readers may be able to build their own index from key entries, but SumatraPDF relies on the one in the main file being a normal default one.

Thanks for that good info. Good to known.