How to stop camera from jumping when player jumps

Questions
2

Something like the following could work:

scene.registerBeforeRender(() => {
    ...

    player.computeWorldMatrix(true);
    const targetPosition = player.getAbsolutePosition();

    /* Set camera's Y position to anything you'd like */
    targetPosition.y = customPositionY;

    camera.target.copyFrom(targetPosition);

    ...
});