Missing pages tree in .chm HTML help viewer

Tested build: 0.4.16-dev-38-g96c65e9

Steps to reproduce:

• Download "Microsoft Debugging Tools for Windows NT 4 and Windows 2000": https://archive.org/details/windbg_NT4_2000_x86_ia64
• Make sure that Wine Gecko engine is installed
• Install "DBG_X86.MSI" package
• Alternatively you can do it without installing the MSI:
  • Open the MSI file with 7-Zip
  • Navigate to "dbgdoc.CABinet"
  • Extract two files "debugger.chi" and "debugger.chm" from there, giving them correct file extensions
• Open "debugger.chm" html help file from where it has been installed or extracted
• Expected result: Page tree is visible
• Observed result: It's missing

Also sometimes it doesn't even show the text of the page.

It works correctly on Windows: