Hey lovelies 
Just in case this helps any Steam users, we found a solution to the mouse offset issue that occurs where:
- You're streaming through Steam using Steam Link (app or device) or Remote Play.
- Your host PC is Windows-based.
- The application being streamed is running on a monitor with scaling >100%.
This issue can commonly affect users streaming to tablets or Steam Decks 
We've already written about this previously here, here, here, and here, but we wanted to compile the key info in one place to help others. We have also written up a post on the Steam Community forums with all the details here, in the hopes that Steam devs will pick up on this and improve the Steam client for Windows.
Basically, it seems that the issue is caused by the Steam client for Windows being DPI unaware. DPI awareness was introduced in Windows 10 Version 1607 (aka Anniversary Update). You can read the technical details about these below:
The below website was what caused us to realise that Steam is DPI unaware and how to force DPI awareness for Steam and any applications running through it:
In short, you can force the Steam client and any relevant DPI-unaware applications to run as DPI aware by setting the default DPI awareness for a process.
e.g.,:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\steam.exe]
"DpiAwareness"="PerMonitorV2"
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\streaming_client.exe]
"DpiAwareness"="PerMonitorV2"
You can do the same for any application you're streaming through Steam: just use the name of the exe.
e.g.,
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\rs2client.exe]
"DpiAwareness"="PerMonitorV2"
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\RuneScape.exe]
"DpiAwareness"="PerMonitorV2"
Hope this helps some folks
BijanCyrus is King Of Love