Steps to duplicate
-Start a creative world.
-Make a portal.
-Place a chest outside the portal.
-Switch to Survival (So that portal transport isn't instant.).
-Just as you get in the portal entrance, open the chest (using either keyboard or mouse), then quickly walk out of portal.
Conclusion The player will not be able to look left or right. Their camera position will be permanently bugged until you pause/unpause the game or open a block UI. Mouse movement appears to be disabled as soon as the player tries to open the chest. Mouse buttons still function. It's always the mouse that is affected, even if you open the chest from the keyboard.
EDIT: This issue affects Minecraft Windows 10 Edition when using a keyboard and mouse.
Linked issues
Comments 3
Also, confirmed on Asus B121 Win10 Pro MCPE 0.12.3 with mouse & keyboard input. After the error message, the mouse is disassociated with the game (moving it doesn't move the player but clicking did work). You can get mouse movement back by interacting with a block UI.
Cannot reproduce on Win10 using touch controls (Asus B121 Win10 Pro) or Android 5.0.1 (Samsung Galaxy S4). Seems to be specific to using mouse keyboard, or maybe other controllers.
Should be tested for other block UIs too.