When Steam windows are on the main screen (using the Windows taskbar) they have several context menu options, from launching specific windows to recently played games. When I move these windows to a secondary monitor with a DF taskbar, all these options disappear. See the screenshots for illustration.
On a related note: when the "Friends" window is opened and pinned to the DF taskbar and then closed, it cannot be re-opened through there anymore. It seems to pin some sort of generic "steam webhelper" process which doesn't open the Friends window.
All of these issues started at the same time a few weeks ago (about 3 weeks I'm guessing), before that all these things were working correctly.
I just tested this out on my end and it seemed to work fine. Could you send me a copy of your troubleshooting info? Here are the steps:
Open the Settings > Troubleshooting tab
Click the "Export Info to File" button
Reply with the file attached
I was able to reproduce the friends list issue on my end, but it seems to happen on the Windows taskbar as well so this might be a Steam thing. Does the issue persist for you on the Windows taskbar as well?
