We use compiled html help (.CHM files) and the HtmlHelp API to provide help for users of our Microsoft Excel add-in. But this this week we've suddenly had a few customers complain that the help doesn't open. They click the link and nothing happens.
Testing this, I can see calls to the HtmlHelp function now return a HWND of NULL. This still works fine in older versions of Excel that we support, the help appears and we can the HWND of the help window. So, because it works in older versions I can only assume this is a change in Office 365, rather than a Windows 10 change. The CHM files themselves can still be opened using Windows File Explorer and the same API calls in a standalone application works fine.
Are any other add-ins developers having this problem? Is this to prevent security flaws in the browser used by HtmlHelp? I would move to standard html help and display it in a browser, but we have a few interactive tutorials that send messages back to our application to demonstrate certain steps. That wouldn't be possible with browser based help. Anyone else having this issue? Is this an official change from Microsoft?