fix(web): fade the chain where it runs under the fold

The list is given whatever height the board has left, which is never a
whole number of rows, so the last word was sliced square by the bottom
edge — which reads as broken rather than as scrollable, and left the
history looking cut off rather than continued.

That row now fades out, and only while something really is below it: at
the true end of the chain there is nothing to promise. Scrolling settles
on whole rows rather than mid-card, and the list keeps its scrolling to
itself instead of carrying on into the page once it reaches the end.

web/src/lib/components/ChainHistory.svelte

@@ -26,6 +26,34 @@
 		if (!list || list.scrollTop > FOLLOW_PX) return;
 		list.scrollTo({ top: 0, behavior: scrollBehavior() });
 	});
+
+	/**
+	 * Whether there is chain below the fold. The list is given whatever height
+	 * the board has left, which is never a whole number of rows, so the last
+	 * one is part in and part out of view — and a card sliced by the bottom
+	 * edge reads as broken rather than as scrollable. It is faded out instead,
+	 * but only while there is really something under it: a fade at the true
+	 * end of the chain would be saying the same thing about nothing.
+	 */
+	let more = $state(false);
+
+	function measure() {
+		if (!list) return;
+		more = list.scrollHeight - list.scrollTop - list.clientHeight > 4;
+	}
+
+	// Re-measured on the three things that change the answer: a new word, the
+	// reader scrolling, and the box being resized — which is what an opening
+	// keyboard, a rotation and a meaning panel unfolding all are.
+	$effect(() => {
+		game.state.chain.length;
+		game.state.expanded.length;
+		if (!list) return;
+		measure();
+		const observer = new ResizeObserver(measure);
+		observer.observe(list);
+		return () => observer.disconnect();
+	});
 </script>
 
 <section class="chain" aria-label={t.chainTitle}>
@@ -36,7 +64,7 @@
 		<!-- role, because list-style: none takes the list semantics away in
 		     Safari with VoiceOver, and "3 trong 24" is most of what the chain
 		     tells somebody listening to it. -->
-		<ol class="rows" role="list" bind:this={list}>
+		<ol class="rows" class:more role="list" bind:this={list} onscroll={measure}>
 			{#each entries as entry, index}
 				<!-- The panel id comes from the row's place in the chain, not the
 				     word, so two rows can never share one. -->
@@ -140,6 +168,26 @@
 		min-height: 4.5rem;
 		overflow-y: auto;
 		list-style: none;
+		/* Rows settle whole rather than half in and half out of view: the
+		   list is a transcript to read back, and a card sliced by the bottom
+		   edge reads as broken rather than as scrollable. Proximity, not
+		   mandatory — a row with its meaning open can be taller than the
+		   window that holds it. */
+		scroll-snap-type: y proximity;
+		/* The list keeps its own scrolling to itself rather than carrying on
+		   into the page behind it once it reaches the end. */
+		overscroll-behavior: contain;
+	}
+
+	.rows > li {
+		scroll-snap-align: start;
+	}
+
+	/* Set while there is chain under the bottom edge: the row that is half in
+	   view fades out instead of being cut off square, which is the only thing
+	   on a phone that says this list scrolls. */
+	.rows.more {
+		mask-image: linear-gradient(to bottom, #000 calc(100% - 28px), transparent);
 	}
 
 	.rows > li {