Using Slackware-14.2 with Xfce and GIMP-2.8.18. I've installed the GIMP HTML help files, and linked to them from /usr/share/gimp/2.0/help. In GIMP, Preferences - Help System, User Manual is set to "Use a locally installed copy", and GIMP confirms "There's a local installation of the user manual".

When I try to use Help from inside GIMP, either with the F1 key or with the menu Help > Help, I get an error dialog which says: "GIMP Message. Calling error for procedure 'plug-in-web-browser': No application is registered as handling this file".

Help works in Slackware-14.1, also using GIMP-2.8.18 (Slackware patch). It opens my selected web browser and displays the help index page.

I should point out that in 14.2 "exo-open URL" does work - opens my browser with the given URL. Right-click Open Link over a URL from Xfce Terminal also works.

Any idea why GIMP is unable to open a browser to the help pages in Slackware-14.2?

Quick question:
How did you install gimp-help? From the Slackbuild or from one of gimp's download mirrors?

Downloaded from a GIMP mirror site and unpacked.
It works if I access it directly from my browser, of course, with a file: URL. So it isn't a huge problem that it doesn't work from inside GIMP. But I'm curious why it fails, when it worked under Slackware-14.1.

We have gimp-help package available in SBo: https://slackbuilds.org/repository/1...stem/gimp-help

There is nothing in the slackbuilds.org gimp-help.SlackBuild script that would affect this, and no post-install script. (Yes, I forgot - help 2.6 and older was just an unpack, but starting 2.8 you have to build it from the download. Which I did.)

Is there anyone running Slackware-14.2, and has gimp-help installed (whether from slackbuilds.org or otherwise)? Does it work from inside GIMP with F1 or Help > Help?

It is looking to me like a gvfs issue, but I don't know if it is a problem on my PCs (I tried on 2), or with Slackware-14.2 itself.

Yes, it works fine on my computer using gimp-help from SBo

OK, thanks, that helps. So there is something wrong on my end.

I've narrowed the problem down a lot. From the GIMP script-fu console, I can use this command (similar to what the help function uses, I think):
I found that this works if test.html has no Document Type Declaration (DOCTYPE line), or has a DOCTYPE that specifies HTML. But if it has a DOCTYPE that specifies XHTML (like the GIMP help files) it fails with the same message I get from Help.

Furthermore, I found that exo-open also works or fails (although without any message) on the same files.

So it looks like the underlying code (which I think is gtk_show_uri()) opens and reads the file, and if it sees XHTML, decides there is no application to handle it. But of course, the browser would handle it fine.

I haven't found the fix yet, but I'm getting closer.

This:
shows that the system thinks the MIME type for this file is application/x-extension-html rather than text/html. I don't know why that should be.

The fix follows from that. Use Xfce MIME Type Editor (under Settings) to associate the browser application with the MIME type application/x-extension-html. Or, you can use Thunar, select an HTML file which has an XHTML DOCTYPE, use Open With Other Application..., pick your browser, and check Use as Default for this kind of file. Same result either way.

That fixes both GIMP Help and exo-open.

One more followup: Turns out the underlying cause was (1) some old files in my ~/.local/share/mime/packages directory - perhaps from a 2013 update to firefox, lightning, or seamonkey; and (2) running update-mime-database after installing Slackware-14.2. This created some weird entries in ~/.local/share/mime/mime.cache from those package files, which overrode the system-wide MIME type for some HTML files.

I removed the junk from ~/.local/share/mime/packages, reran update-mime-database, and now it all works as it should without those additional MIME type assocations. "xdg-mime query filetype" now just returns text/html for all the files as it should, and GIMP Help works.