Mouse is captured in windowed-fullscreen game in multimonitor configuration when it should not be.

This happens and was tested in all currently available versions of Proton.

In all games, under WINE and in Windows, so long as the game gives you a cursor, you are able to drag the cursor out of the game window and onto another monitor or your desktop.

Under Proton however, this is not the case. The mouse is trapped in the game window, despite being windowed fullscreen (or "borderless fullscreen" in some games). The mouse should be able to move freely away from the game if the game presents a cursor (such as entering a menu in Warframe or Skyrim) as it does on Windows and in WINE. The only way to currently do this is to Alt+Tab onto another program and then move your mouse out of the game window.

Start by reproducing the issue with a windowed-fullscreen game, a cursor-visible menu, and multiple monitors under Proton. Compare cursor movement with the reported Windows and WINE behavior. Done means the cursor can leave the game window when the game presents a cursor, without requiring Alt+Tab.