If your mouse is doing the following,

  • moving to the top of the screen quickly and sticking there,
  • moving incredibly slowly (turning a fraction of a degree),
  • cursor limited to a small portion of your screen in menus / pop-up dialogues,
  • and/or not clicking any buttons in menus, pop-up dialogues, and cameras with cursor input (i.e. turret fortress commander),

then please do the following to correct it:

  • In your Steam games menu, right-click Half-Life then click Properties
  • Select the Betas tab
  • Select "Beta - Beta" in the dropdown
  • Run Half-Life once to let it update
  • Now run Sven Co-op
  • Enter "version" in your console and read the line starting with "Exe build"
  • If the 4-digit number at the end is lower than 5986, then you're doing it wrong
  • Enter "m_rawinput 0" in your console as Sven Co-op 4.7 is not built to handle the engine's new raw mouse input handling yet

Related links:

https://github.com/ValveSoftware/halflife/issues/432
https://github.com/ValveSoftware/halflife/issues/611
https://github.com/ValveSoftware/halflife/issues/653
https://github.com/ValveSoftware/halflife/issues/801
...probably plenty more.