Material-ui: <Slider track="inverted"> does not highlight the currently-selected mark

[seansfkelley]

• [x] I have searched the issues of this repository and believe that this is not a duplicate.

Summary 💡

When using discrete marks with the forward slider, marks are highlighted if the slider is at or above their value.

When using discrete marks with the inverted slider, marks are highlighted if the slider is strictly above their value.

Examples 🌈

Using sliders from the slider docs.

A forward slider at exactly 37 degrees:

An inverted slider at exactly 37 degrees:

An inverted slider at maximum of 100 degrees has no highlights:

Motivation 🔦

In my case, I have a Low/Medium/High slider, and it's being used to set a minimum threshold for importance. The threshold is inclusive, but the visuals don't suggest that. It makes sense to be inclusive in this case, because it would not make sense for an exclusive slider to be on High as nothing would match that filter.

[mbrookes]

@seansfkelley What do you think of this diff?

diff --git a/packages/material-ui/src/Slider/Slider.js b/packages/material-ui/src/Slider/Slider.js
index 99663febd..0ec32c258 100644
--- a/packages/material-ui/src/Slider/Slider.js
+++ b/packages/material-ui/src/Slider/Slider.js
@@ -741,16 +741,19 @@ const Slider = React.forwardRef(function Slider(props, ref) {
       {marks.map(mark => {
         const percent = valueToPercent(mark.value, min, max);
         const style = axisProps[axis].offset(percent);
-
         let markActive;
         if (track === false) {
           markActive = values.indexOf(mark.value) !== -1;
         } else {
-          const isMarkActive = range
-            ? mark.value >= values[0] && mark.value <= values[values.length - 1]
-            : mark.value <= values[0];
           markActive =
-            (isMarkActive && track === 'normal') || (!isMarkActive && track === 'inverted');
+            (track === 'normal' &&
+              (range
+                ? mark.value >= values[0] && mark.value <= values[values.length - 1]
+                : mark.value <= values[0])) ||
+            (track === 'inverted' &&
+              (range
+                ? mark.value <= values[0] || mark.value >= values[values.length - 1]
+                : mark.value >= values[0]));
         }

         return (

[oliviertassinari]

👍 for adding a regression test too

[seansfkelley]

I haven't thought through any potential edge cases in that diff, but using non-strict comparison operators for both kinds seems to be the key change, so looks good to me! Thanks for the quick turnaround.

[mbrookes]

No worries, would you like to submit a PR?

[ulises-lara]

I can take care of this, if I may.
I'll be working to submit this PR.