AgX: wrong history display for pivot relative exposure

Is there an existing issue for this?

• I checked and did not find my issue in the already reported ones

Describe the bug

Root cause:

• The module stores the x coordinate of the pivot in the pivot_x property, presented via the curve/pivot relative exposure slider
• It modifies (and on each change, recalulates) the offset and scale of the slider, so the user is presented with an EV value
• In the history change popup, the previous value of pivot_x is interpreted using the current offset and scale.

For example, starting from black/white -10 EV -> 6.5 EV, pivot at mid-grey (0 EV shift), and changing the black/white relative exposure to -8 EV -> 5 EV, and setting the pivot shift to 1.5 EV, then hovering over the history item 12 will show the original pivot exposure as -0.12 EV, not as 0 EV, because the old pivot_x is interpreted using the new scale and offset:

Resetting the sliders and hovering over history item 13 will then show the pivot was moved from 2.06 EV to 0 EV (not the correct 1.5 EV to 0 EV):

Hovering over the previous entry, number 12 (which previously showed -0.12 EV -> 1.5 EV), shows the pivot was shifted from 0 EV to 2.06 EV:

The field stores the pivot_x value (0..1, x coordinate of the curve's pivot) and not the EV value, and changes visualisation (offset and scale) so we don't have to modify hard limits (which was the previous solution, using those hard limits to enforce the pivot is between black and white relative exposure). This was done to address https://github.com/darktable-org/darktable/pull/19642#discussion_r2485896335. @dterrahe

This is a relatively minor issue, and is probably preferable to messing with hard limits.

Steps to reproduce

See above

Expected behavior

the history should be displayed correctly

Logfile | Screenshot | Screencast

No response

Commit

No response

Where did you obtain darktable from?

darktable.org / GitHub release

darktable version

7035503eb7283e1e8

What OS are you using?

Linux

What is the version of your OS?

OS independent

Describe your system

No response

Are you using OpenCL GPU in darktable?

None

If yes, what is the GPU card and driver?

No response

Please provide additional context if applicable. You can attach files too, but might need to rename to .txt or .zip

No response

Start by locating the module that stores pivot_x and the history change popup that displays its previous value. Reproduce the black/white and pivot-relative-exposure sequence described in the issue, then trace how the stored pivot_x is interpreted before and after slider offset and scale changes. Done means history entries display the correct previous and current EV values.