Connecting to localhost  

Full story: I would like to share my use case - which is possibly a bit unusual:
I am regularly giving workshops and training courses where I switch between presenting PowerPoint slides and demonstrating something on my PC (Windows 10 notebook with Intel standard graphics). I like to use PowerPoint's presenter view, to be able to see my speaker notes and preview the next slide, but this only works with extended screens. In a classroom this is usually fine, since I can extend my desktop to the projector or wall screen. However, when I demonstrate something (like the usage of a tool), I have to switch to duplicating the desktop and then back again. This then forces me to stop and restart the presentation. Not a big issue, just a little bit annoying.
Since the Corona virus pandemic, I am now working from home (based in Germany), which changed things somehow. I am now giving webinars with Zoom or Skype for Business. These tools allow to share a program, the whole desktops or one screen with the remote participants. So to do the scenarios described above, I need a second screen - otherwise I cannot use presenter view. And this is where Spacedesk comes in:
I am using the HTML5 viewer app to connect to localhost - the Windows native viewer unfortunately prevents connections to localhost. Within the HTML5 viewer, I set a screen resolution that is smaller than my main screen and use a browser extension to resize the window properly. This way, I have a virtual second screen running in a window. When presenting, I can now use presenter view and the slide is shown on the virtual second screen. This virtual second screen is shared via Zoom or Skype, which also allows me to look something up etc. without interrupting the shared view for the participants. When I want to demonstrate something, I just move whatever I want to show onto this virtually extended screen. Also I can easily switch between the slides and a demonstration.

Hi @flolll,
Yes, we only allow local host connection on HTML5 Viewer.
With native windows viewer app we don't allow it at the moment.