diff --git a/client_v2/src/lib/components/ColorEditor.svelte b/client_v2/src/lib/components/ColorEditor.svelte
index cb2206b6..67b173bc 100644
--- a/client_v2/src/lib/components/ColorEditor.svelte
+++ b/client_v2/src/lib/components/ColorEditor.svelte
@@ -96,9 +96,11 @@
 	}
 	// Transparent-to-opaque gradient over a checkerboard, so the slider previews
 	// what it controls. Falls back to grey until the row holds a real colour.
+	// `border-box no-repeat` keeps the gradient from tiling a hairline of full
+	// colour under the border at the transparent end — see ALPHA_CHECKER.
 	function alphaTrack(h: string): string {
 		const rgb = normalizeHex(h)?.slice(0, 6) ?? '9AA0A6';
-		return `background:linear-gradient(to right,#${rgb}00,#${rgb}FF),${ALPHA_CHECKER}`;
+		return `background:linear-gradient(to right,#${rgb}00,#${rgb}FF) border-box no-repeat,${ALPHA_CHECKER}`;
 	}
 	function addColor() {
 		list.push('');
diff --git a/client_v2/src/lib/utils/color.ts b/client_v2/src/lib/utils/color.ts
index 5c10e5d5..dc1b3338 100644
--- a/client_v2/src/lib/utils/color.ts
+++ b/client_v2/src/lib/utils/color.ts
@@ -45,9 +45,17 @@ export function withAlpha(h: string, alpha: number): string {
  * transparent instead of as a different color. Deliberately theme-independent
  * (the usual white/grey of every color picker) and composited over white, which
  * also previews how the filament looks against a printed label.
+ *
+ * Every layer is anchored to the border box. A `background` shorthand resets
+ * `background-origin` to the padding box, so on a bordered element the layers
+ * would be sized to the padding box but painted out under the border — and
+ * since backgrounds repeat, the strip under the border shows the far edge of
+ * the next tile. On a colour gradient that is a hairline of full-opacity colour
+ * down the transparent end, visible wherever the border doesn't perfectly cover
+ * it (Firefox at fractional display scaling).
  */
 export const ALPHA_CHECKER =
-	'conic-gradient(#cfcfcf 0 25%, #0000 0 50%, #cfcfcf 0 75%, #0000 0) 0 0/6px 6px, #fff';
+	'conic-gradient(#cfcfcf 0 25%, #0000 0 50%, #cfcfcf 0 75%, #0000 0) 0 0/6px 6px border-box, #fff';
 
 export function hexRgb(h: string): [number, number, number] {
 	let s = (h || '#888').replace('#', '');