Is there any way I could hold down the mouse button whilst being tabbed out? My goal is to be able to have multiple windows running at the same time on 1 PC, all of them holding down W and LMB.
Nope, that was changed in 1.13. Minecraft now uses a standardized implementation of Fullscreen mode that does not lose track of things like button states when focus changes. You can perhaps work up a script via AutoHotkey, but I've heard mixed results from that.
Is there any way I could hold down the mouse button whilst being tabbed out? My goal is to be able to have multiple windows running at the same time on 1 PC, all of them holding down W and LMB.
Nope, that was changed in 1.13. Minecraft now uses a standardized implementation of Fullscreen mode that does not lose track of things like button states when focus changes. You can perhaps work up a script via AutoHotkey, but I've heard mixed results from that.