F5 and looking up no longer break a page

• Fixed: pressing F5 with a fullscreen page or an overlay open pulled the camera off the page. The hotbar came back and the pointer drifted. The camera now stays put in every perspective.
• Fixed: in F5's front view the pointer moved the wrong way. It now follows the mouse in all three perspectives.
• Fixed: an overlay page vanished, with its tint and the world round it, when the player looked up about 36 degrees (or down, in F5's front view). It now stays on screen wherever they look.
• Tested on Paper, Spigot and Folia, on 1.21.11, 26.1.2, 26.2 and 26.3, and with 250 ms of lag each way.