Toolbox "Change FG tool", sometimes LCh sliders can't be moved independently
For GIMP-2.99, when using the FG/BG tool to dial in a color, when starting with the LCh color L=C=h=0 (solid black, R=G=B=0):
Dialing in L=50 (leaving the other channels alone) makes the LCh hue reset itself to h=142-ish.
Resetting h to 0 and dialing in C=20, the hue again reset itself to a hue other than 0, on my last try it reset to 4.
Going back and changing "L" makes the other sliders change again.
Similar oddities happen in GIMP-2.10, so this likely isn't a result of code changes for "anyrgb".
This "resetting" of the hue when changing the Lightness and Chroma absolutely shouldn't happen. And modifying L shouldn't also make one or both of the other LCh sliders move. It should be possible to start with solid black L=C=h=0, and dial in L=50, with C and h staying at 0. And then it should be possible to set C to a number greater than 0, and still have the hue stay at 0. In other words, modifying any one slider should not cause the other sliders to move.
The problem also affects the "FG/BG Color" docker sliders, though the exact numbers that pop up vary from the Toolbox Change Foreground tool. Also, in the FG/BG tool, sometimes moving the LCh sliders to be zero makes one or more of the RGB channels read as negative zero.
This slider resetting might be related to the problem that blending a top color layer using LCh Chroma over a bottom gray layer sometimes produces unexpected hue shifts. Note in the 2.10 screenshot below, the hue shift just from the blend - the Sample Points show the blended RGB and LCh. The color picker is set to only read the actual color layer rather than the merged result. Both color readouts should have the same hue and Chroma, but notice the blended hue is different from the hue of the actual layer:
This is on Debian Sid, updated today, and using babl/GEGL/both versions of GIMP also updated today.