This conversation has been merged. Please reference the main conversation: Lightroom 2015 curves bug
If I grab the white level (top right gray point) in the curve and drag, my mouse disappears, and as I drag around, I quickly "run out of room" to drag, and I can no longer drag the point to the right. When I let go of the mouse button, my mouse reappears on the screen, and it turns out to be on the edge of my screen. If you play with this control, you will quickly see that Adobe has disconnected the mouse position with the dot position (presumably to allow finer control), but when you run into the edge of the screen, you can't move the dot any farther in that direction.
If you move the right edge of your window away from the edge of the screen, then you regain more control (although you can still "run out of space") but at the cost of a smaller image to judge the effects of your edits. There is some hysteresis so that as you drag to the right, and then drag back to the left, the mapping of cursor location to point on the curve keeps moving farther and farther to the right, ensuring that sooner or later you will run past the edge of the screen.
I so much prefer Photoshop's curve control, where you can use the arrow keys and can type in numbers to set the x & y position.
Lightroom CC 2015.7. Macintosh iMac 5K.