👍

Looks fine.
Glad you didn't change the 3 first values (so, no shift in settings - no book re-rendering or visual changes for users) and only added new values.

(It will just be a bit strange for people who did customized the 3 first ones in default.persistent.lua - probably they increased them, possibly beyond the 4th and 5th).

Nothing to do with this PR, just brainstorming (even for a possible personal hack):
On my GloHD, a refresh on a previously solid black area makes the underlying test grayer/thiner instead of black.
So, with these black progress bars like contrast and this new one, I usually follow quiting the bottom menu with a small diagonal swipe to full refresh.
I don't think it's worth doing a full refresh when quiting a bottom config dialog that would include these black bars - might not be needed on other devices.
Just wondering if there's a visual idea for such progress bar without the big solid blacks - while still as readable as to what is selected. I can't think of any :|

I don't think it'd suffer in usability from using the same gray as the toggle switches, if that's any better for ghosting without a flash.

PS I'll fix the unit tests later.

Well, solid gray areas auto-flash-ui themselves on my GloHD :) and I really don't like that either (I've converted some of our gray lines to black in my own patch - separators in the menu were flashing and that did bother me).
But ok, I don't mind that much the gray toggles, so I'll try that progress bar dressed in gray.

solid gray areas auto-flash-ui themselves

I might be overlooking something (especially since I haven't looked at the code and am therefore unaware of its comments) but I'd say either so should black or neither should gray, depending on which behavior is best?

No worry with our code. It's the (GloHD at least) hardware that does this auto-flash with grey (it's not a single transition: it goes from white to black, and they from black to grey - or something like that - no matter what we use in our code).

Made it slightly prettier:

I don't think it'd suffer in usability from using the same gray as the toggle switches, if that's any better for ghosting without a flash.

It's actually far better in grey on my GloHD (because of the HW auto-flash, which leaves no ghosting at all):

A bit too monotonous though :)

In case one wantd to try it on some other devices:

--- a/frontend/ui/widget/buttonprogresswidget.lua
+++ b/frontend/ui/widget/buttonprogresswidget.lua
@@ -43,13 +43,15 @@ function ButtonProgressWidget:update()
     local button_margin = Size.margin.tiny
     local button_padding = Size.padding.button
     local button_bordersize = Size.border.button
-    local preselect
+    local preselect, background
     local button_width = math.floor(self.width / self.num_buttons) - 2*button_padding - 2*button_margin - 2*button_bordersize
     for i = 1, self.num_buttons do
         if self.position >= i then
             preselect = true
+            background = Blitbuffer.COLOR_GREY
         else
             preselect = false
+            background = Blitbuffer.COLOR_WHITE
         end
         local button = Button:new{
             text = "",
@@ -60,6 +62,7 @@ function ButtonProgressWidget:update()
             enabled = true,
             width = button_width,
             preselect = preselect,
+            background = background,
             text_font_face = self.font_face,
             text_font_size = self.font_size,
             callback = function()

Although I vaguely recall having removed the flashing from the bottom menu to avoid bad interactions with the top menu ;).

Indeed, no flash on the bottom menu.

Holding on margin title (to have help text) now crashes with:

./luajit: frontend/ui/data/optionsutil.lua:88: attempt to get length of field 'toggle' (a nil value)
stack traceback:
        frontend/ui/data/optionsutil.lua:88: in function 'name_text_hold_callback'
        frontend/ui/widget/configdialog.lua:277: in function 'hold_callback'
        frontend/ui/widget/button.lua:241: in function 'handleEvent'
        frontend/ui/widget/container/inputcontainer.lua:255: in function 'handleEvent'

(No crash when holding on the Contrast title)