P: Tone curve still broken in
The bug that I reported here has not been squashed in the latest Lightroom.
The problem is hysteresis in the response of points on the tone curve control when following mouse movements.
If you drag a point (say, the white point) in the Tone Curve and move it back and forth, after a few drags you will not be able to put the point back in the upper right corner of the response panel.
The underlying cause is that (a) the point does not move directly with the cursor (LR hides the cursor so you can't see this, but when you let go of the mouse button you will see that the cursor does not align with the point, (b) as you drag, the relationship between the cursor position and the point changes, when the point hits the right edge of the box and the cursor continues to the right, then when you move the cursor back to the left it does not "pick up" the point at the same location that it "left" the point, and (c) the (invisible) cursor is limited by Mac OS to stay on the screen.
The work-around (which is not perfect), is to make your Lightroom window smaller so that there is more space beyond the right edge of the window, so the hysteresis effect doesn't show up as quickly. If you have 2 screens, you can move the Lightroom window to the left screen, which effectively solves this problem.
I assume that this problem occurs for all four edges of the Tone Panel, but because the Tone Panel is on the right hand side of the Lightroom window, the effect is seen mostly when trying to move a point on the tone curve to the right hand side of the panel.
If this is not clear, let me know, and I'll see if I can post a video.
[Edit -- I just confirmed this bug for the top edge of the Tone Curve as well.]
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