Multi-monitor host streaming to Steam Link

[Arno]

TLDR

When desktop streaming from a multi-monitor host, the Steam Link (hardware) doesn't display a sane region of the host's desktop (even when the setup works fine when the host has a single monitor).

ISSUE

First off let me be clear that this is about desktop streaming. The host machine doesn't have any games installed on it, and may never. I'm only using Steam for desktop sharing (as opposed to some other solution like XDMCP or VNC) because I already have the Steam Link in the other room, for use with other hosts. If the Steam Link hardware supported other protocols (XDMCP, NX, VNC, RDP, etc.) I would probably be using those for this particular host instead.

I will mark a post as an answer if I get one; even if the answer is "Steam doesn't support that" as long as it comes from an authoritative source. If you don't see an answer marked, I still consider the issue unresolved; I'm taking the time to point this out because posts like this sometimes attract low-value answers and I don't want to spend time responding to them just so it doesn't appear like they solved the problem. Sorry, that's just the way I feel about it. :) That said, feel free to ask for any clarifying details about the hardware/software configuration.

I'm not currently running the beta Steam client or Steam Link firmware, but I'm not opposed to trying it if there's a specific reason why it might help (e.g. if this issue is known to be addressed in a later release than the current stable). Otherwise I don't want to mess with what I know is a stable setup as long as I disconnect this second monitor.

Everything with this setup works fine as long as the host only has a single monitor connected. When you connect a second monitor, however, Steam doesn't seem to be able to differentiate between different monitors and can only display the entire (aggregate) desktop, either zoomed out to fit on the client's single monitor, or in parts.

The monitor configuration, in case it matters, is thus:

• Host primary display: 1080p HDMI connection, landscape orientation
  
• Host secondary display: 1080p DP connection, portrait orientation
  
• Client display: 1080p HDMI connection, landscape orientation

The host's overall desktop is shaped like a capital "T" that's been rotated 90 degrees counter-clockwise so it looks like "|-" (the portrait secondary monitor is to the left of the landscape primary). When the Steam Link's display "zooms out", you can see the black/empty areas that have been inserted to make that "T" shape into a regular rectangle.

OS AND VERSIONS

• Host OS: Kubuntu 18.04 LTS (KDE desktop environment)
  
• Steam version: Current on updates, not running the beta version (about box says "Package versions 1573780595")
  
• Steam Link firmware: Current on updates as far as I know, not running the beta version ("System Information" says it's build 698)

STEPS TO REPRODUCE

Everything works fine as long as the host only has one monitor connected. When I connect a second monitor to the host, this is what happens; it still mostly works, but...

1. Launch Steam on the host
   
2. Select the host from the Steam Link's UI
   
3. Observe that Big Picture launches, displays, and reacts to input correctly
   
4. Choose "Exit Big Picture"
   
5. Observe the issue when (part of) the desktop is displayed

OBSERVED RESULT

An area (what looks to be a 1920x1080 area, but don't quote me) of the host's total desktop area is displayed, but not the area you'd want. If you move the mouse to certain areas, the view "zooms out" and you can see the entire desktop (both screens' worth).

EXPECTED/DESIRED RESULT

The Steam Link would present the host's primary monitor (solely, and in its entirety). I realize this might mean that windows on other monitors would be permanently inaccessible, but that's a situation I can work around and live with; the current behaviour is unusable in all use cases, unfortunately. For example, if I had the whole primary monitor then I could use KDE's window management controls to bring any other windows to that primary desktop.

WHAT I HAVE TRIED

• Disconnecting/restarting the Steam Link
  
• Physically disconnecting/reconnecting the secondary monitor
  
• Both settings of "Settings > Remote Play > Advanced Host Options > Change desktop resolution to match streaming client"