You are absolutely right, the solid magenta is disconcerting to look at. Changing the gamut marker color to black does make it easier to look at, at least for me it’s a better color choice.
Part of the problem is that there are still 8-bit integer variables in the “color the slider and panel” code, so the magenta (or whatever “out of gamut” color you choose) kicks in before it should, thereby covering too much of the sliders. This is something that I think will be fixed before 2.10 is released.
A problem with dithering or blending to make the transition less harsh is that the user would no longer know precisely when/where the colors go out of gamut. Having the option to work with out of gamut colors opens up some very nice editing possibilities, but creates problems when not intended or anticipated. So that sharp line “don’t go past here” serves a purpose.
I’ve been thinking about what might make it easier to use the LCH color tools. An outline marking “in and out of gamut” might be easier to look at than the current magenta color overlay.
It would be really nice to have an actual LCH color wheel, which would necessarily be quite different from an HSV color wheel. I’ve been working on a description of what such a wheel might look like and do, and can share what I’ve come up with if anyone is interested, and of course ideas/input from other GIMP users would be nice.