When the Fullscreen Windowed (Borderless) key is unbound, pressing a media button (play, volume etc) on some keyboards will toggle Fullscreen on/off, causing really glitchy behaviour when increasing volume.

Versions

• FullscreenWindowed-1.12-1.6.0
• Forge-1.12.2-14.23.5.2847

I was pretty frustrated by this, but for some reason, just setting a keybind for the vanilla fullscreen key in the controls options to any key solves it. Having it unbound is what causes this issue, for whatever reason.