Simplifying the structure of s-expandable and removed outdated styles

lib/css/components/expandable.less

@@ -1,104 +1,20 @@
-// see http://stackoverflow.com/a/43965099 for how this works:
-
-// Notes on the clip-path stuff: What we would really like is overflow: hidden during the transition,
-// but not when the element is expanded. Unfortunately, although the CSS spec provides for it, there's
-// no browser support yet for transitioning non-interpolatable properties, and therefore we cannot say
-// "change overflow to visible *after* the transition".
-//
-// So we use clip-path: polygon(0% 0%, 100% 0%, 100% 100%, 0% 100%), which is essentially the same
-// as overflow: hidden, but unlike overflow, clip-path is interpolatable -- but only if the opposite state
-// is also a polygon, and has the same vertice count. So our version of overflow: visible is a rectangle
-// of 2x2 million pixels, which should be enough for everybody (TM).
-//
-// At the time of writing, clip-path works in Chrome, Firefox Beta, and Safari (with vendor prefix).
 //
-// In browsers that do not support this yet, we have to have another way to prevent the expandable content
-// from being visible below the bottom edge during the transition, and the best I could come up with is
-// transitioning to scaleY(0) with a timing function that's faster than the height-reducing transitions.
-// This does kinda look like a deliberate effect, even more so if we also transition to transparency
-// (which we therefore do), so I feel it's an okay fallback.
+//  STACK OVERFLOW
+//  EXPANDABLE
 //
-// We still set overflow to hidden in the collapsed state (which also applies to the collapsing transition),
-// we just can't do that for the expanding transition.
+//  This CSS comes from Stacks, our CSS & Pattern library for rapidly building
+//  Stack Overflow. For documentation of all these classes and how to contribute,
+//  visit https://stackoverflow.design/
 //
-// A major drawback of clip-path is that even though the content is clipped (not visible), it will still
-// contribute to the document height. This caused a jumping around of the scrollbar (and possibly even
-// of the viewport) when the expandable was at the bottom of the page, because once the negative bottom
-// margin of the -expandable-group exceeds the actual height, any additional pixels were added on to the
-// bottom. The fix is to set the -expandable-group's flex alignment to flex-start, which forces the
-// (no longer visible) element itself to remain at the top, thereby forcing the excess pixels to be added
-// above the top, not below the bottom. And because extending content above the document top will not do
-// anything to the document height, there is no jumping during the transition.
-
-@stacks-internals-s-expandable-transition-duration: 100ms;
-
-// Per the answer referenced above, the component can only guarantee smooth transitions if above a minimum
-// height and can only guarantee the element will be hidden is below a maximimum height.
-// The minimum height has been set at 10px because that's below the height of a single line of text in an s-description.
-@stacks-internal-s-expandable-min-expected-height: 10px;
-@stacks-internal-s-expandable-max-expected-height: 1500px;
 
 .s-expandable {
-    display: flex;
-    -webkit-clip-path: polygon(-1000000px -1000000px, 1000000px -1000000px, 1000000px 1000000px, -1000000px 1000000px);
-    clip-path: polygon(-1000000px -1000000px, 1000000px -1000000px, 1000000px 1000000px, -1000000px 1000000px);
-    align-items: flex-start; // see comment above
-    transition: clip-path 0s @stacks-internals-s-expandable-transition-duration, -webkit-clip-path 0s @stacks-internals-s-expandable-transition-duration;
-
-    &:after {
-        content: '';
-        -ms-flex-preferred-size: 0;
-        flex-basis: 0;
-        height: @stacks-internal-s-expandable-min-expected-height;
-        max-height: 0;
-        transition:
-            height @stacks-internals-s-expandable-transition-duration linear,
-            max-height 0s @stacks-internals-s-expandable-transition-duration linear;
-    }
-}
-
-.s-expandable--content {
-    -ms-flex-preferred-size: 100%;
-    flex-basis: 100%;
-    max-height: 1000000px;
-    margin-bottom: 0;
-    -webkit-transform-origin: 0 0;
-    transform-origin: 0 0;
-    transition:
-        margin-bottom @stacks-internals-s-expandable-transition-duration cubic-bezier(0, 0, 0, 1),
-        transform @stacks-internals-s-expandable-transition-duration cubic-bezier(1, 0, 1, 1),
-        opacity @stacks-internals-s-expandable-transition-duration cubic-bezier(1, 0, 1, 1);
-}
-
-.s-expandable:not(.is-expanded) {
+    height: 0;
+    opacity: 0;
+    visibility: hidden;
     overflow: hidden;
-    -webkit-clip-path: polygon(0% 0%, 100% 0%, 100% 100%, 0% 100%);
-    clip-path: polygon(0% 0%, 100% 0%, 100% 100%, 0% 100%);
-    transition: none;
-
-    .s-expandable--content {
-        visibility: hidden;
-        max-height: 0;
-        margin-bottom: -@stacks-internal-s-expandable-max-expected-height;
-        opacity: 0;
-        -webkit-transform: scaleY(0);
-        transform: scaleY(0);
-        transition:
-            margin-bottom @stacks-internals-s-expandable-transition-duration cubic-bezier(1, 0, 1, 1),
-            visibility 0s @stacks-internals-s-expandable-transition-duration,
-            max-height 0s @stacks-internals-s-expandable-transition-duration,
-            transform @stacks-internals-s-expandable-transition-duration cubic-bezier(0, 1, 1, 1),
-            opacity @stacks-internals-s-expandable-transition-duration cubic-bezier(0, 1, 1, 1);
-        @supports ((-webkit-clip-path: polygon(0% 0%, 100% 0%, 100% 100%, 0% 100%)) or (clip-path: polygon(0% 0%, 100% 0%, 100% 100%, 0% 100%))) {
-            opacity: 1;
-            -webkit-transform: none;
-            transform: none;
-        }
-    }
-
-    &:after {
-        height: 0;
-        max-height: @stacks-internal-s-expandable-min-expected-height;
-        transition: height @stacks-internals-s-expandable-transition-duration linear;
-    }
 }
+.s-expandable.is-expanded {
+    height: auto;
+    visibility: visible;
+    opacity: 1;
+}