diff --git a/packages/block-editor/src/components/block-list/content.scss b/packages/block-editor/src/components/block-list/content.scss
index 3969a0c917660..db5807cd0a6a4 100644
--- a/packages/block-editor/src/components/block-list/content.scss
+++ b/packages/block-editor/src/components/block-list/content.scss
@@ -458,11 +458,15 @@ _::-webkit-full-page-media, _:future, :root .has-multi-selection .block-editor-b
 .block-editor-block-list__zoom-out-separator {
 	/* same color as the iframe's background */
 	background: $gray-300;
+	// Additional -1px is required to avoid sub pixel rounding errors allowing background to show.
+	margin-left: -1px;
+	margin-right: -1px;
 }
 
 // In Post Editor allow the separator to occupy the full width by ignoring (cancelling out) the global padding.
 .has-global-padding > .block-editor-block-list__zoom-out-separator,
 .block-editor-block-list__layout.is-root-container.has-global-padding > .block-editor-block-list__zoom-out-separator {
 	max-width: none;
-	margin: 0 calc(-1 * var(--wp--style--root--padding-right)) 0 calc(-1 * var(--wp--style--root--padding-left)) !important;
+	// Additional -1px is required to avoid sub pixel rounding errors allowing background to show.
+	margin: 0 calc(-1 * var(--wp--style--root--padding-right) - 1px) 0 calc(-1 * var(--wp--style--root--padding-left) - 1px) !important;
 }