fix: related content overflow and horizontal scroll issues (#1305)

* fix: resolve related content card overflow and page horizontal scroll

* fix: prevent related content overflow on mobile while preserving shadow space

* fix: align related content cards and prevent shadow clipping

Expands the left margin of the related content carousel to match the main article, preserving the full card shadow. Adds a visual mask gradient to gracefully fade out the right edge.

* feat: add mask image also to left side of subsection list, which is activated when scrolling

* feat: fade out right mask image near the end of horizontal scroll

---------

Co-authored-by: Jimmy Cai <jimmy@cai.im>

assets/scss/partials/layout/list.scss

@@ -44,24 +44,79 @@
     }
 }
 
+@property --left-edge-fade-size {
+    syntax: "<length>";
+    inherits: false;
+    initial-value: 0px;
+}
+
+@property --right-edge-fade-size {
+    syntax: "<length>";
+    inherits: false;
+    initial-value: 64px;
+}
+
 .subsection-list {
     --shadow-overlap-x: calc(var(--shadow-l2-blur) + 4px);
     --shadow-overlap-y: calc(var(--shadow-l2-blur) + var(--shadow-l2-y) + 4px);
+    --edge-fade-size: 64px;
+    --left-edge-fade-size: 0px;
+    --right-edge-fade-size: var(--edge-fade-size);
 
     overflow-x: auto;
 
+    /// Limit the horizontal overlap to the container padding to prevent page overflow on mobile.
+    /// On desktop, it still allows the full shadow to be visible.
+    --overlap-x: min(var(--shadow-overlap-x), var(--container-padding));
+
     /// These values are computed from the shadow of the article-list--tile (which uses shadow-l1 and shadow-l2)
-    /// so that the shadow is not cut off
+    /// so that the shadow is not cut off, while staying within the viewport on small screens.
-    margin-left: calc(var(--shadow-overlap-x) * -1);
+    /// Clamp both sides to prevent page-level horizontal overflow.
-    margin-right: calc(var(--shadow-overlap-x) * -1);
+    margin-left: calc(var(--overlap-x) * -1);
+    margin-right: calc(var(--overlap-x) * -1);
     margin-bottom: calc(var(--shadow-overlap-y) * -1);
 
+    /// Smooth fade on both horizontal edges to avoid hard cuts where shadows are clamped.
+    --subsection-mask-image: linear-gradient(
+        to right,
+        transparent 0%,
+        rgb(0 0 0 / 0.06) calc(var(--left-edge-fade-size) * 0.12),
+        rgb(0 0 0 / 0.2) calc(var(--left-edge-fade-size) * 0.3),
+        rgb(0 0 0 / 0.45) calc(var(--left-edge-fade-size) * 0.52),
+        rgb(0 0 0 / 0.72) calc(var(--left-edge-fade-size) * 0.76),
+        black var(--left-edge-fade-size),
+        black calc(100% - var(--right-edge-fade-size)),
+        rgb(0 0 0 / 0.72) calc(100% - (var(--right-edge-fade-size) * 0.76)),
+        rgb(0 0 0 / 0.45) calc(100% - (var(--right-edge-fade-size) * 0.52)),
+        rgb(0 0 0 / 0.2) calc(100% - (var(--right-edge-fade-size) * 0.3)),
+        rgb(0 0 0 / 0.06) calc(100% - (var(--right-edge-fade-size) * 0.12)),
+        transparent 100%
+    );
+    -webkit-mask-image: var(--subsection-mask-image);
+    mask-image: var(--subsection-mask-image);
+
+    // CSS-only: left fade ramps in quickly once horizontal scrolling starts,
+    // and right fade turns off near the end of horizontal scroll.
+    animation: subsection-left-fade linear both, subsection-right-fade linear both;
+    animation-timeline: scroll(self inline);
+    animation-range: 1px 24px, 0% 100%;
+    animation-timing-function: ease-out, linear;
+
+    [dir="rtl"] & {
+        margin-left: calc(var(--overlap-x) * -1);
+        margin-right: calc(var(--overlap-x) * -1);
+    }
+    
     .article-list--tile {
         display: flex;
-        padding: var(--spacing-xs) var(--shadow-overlap-x) var(--shadow-overlap-y) var(--shadow-overlap-x);
+        padding: var(--spacing-xs) var(--overlap-x) var(--shadow-overlap-y) var(--overlap-x);
         width: max-content;
         gap: var(--spacing-lg);
 
+        [dir="rtl"] & {
+            padding: var(--spacing-xs) var(--overlap-x) var(--shadow-overlap-y) var(--overlap-x);
+        }
+
         article {
             flex-shrink: 0;
 
@@ -76,3 +131,22 @@
         }
     }
 }
+
+@keyframes subsection-left-fade {
+    from {
+        --left-edge-fade-size: 0px;
+    }
+    to {
+        --left-edge-fade-size: var(--edge-fade-size);
+    }
+}
+
+@keyframes subsection-right-fade {
+    0%,
+    92% {
+        --right-edge-fade-size: var(--edge-fade-size);
+    }
+    100% {
+        --right-edge-fade-size: 0px;
+    }
+}