diff --git a/packages/desktop/src/preload.ts b/packages/desktop/src/preload.ts
index fe139d32..6ddcbd26 100644
--- a/packages/desktop/src/preload.ts
+++ b/packages/desktop/src/preload.ts
@@ -21,7 +21,9 @@ if (process.platform === 'darwin' && window.location.origin === APP_ORIGIN) {
 			top: 0;
 			right: 0;
 			left: 0;
+			box-sizing: border-box;
 			height: 2rem;
+			border-bottom: 1px solid var(--border);
 			content: '';
 			-webkit-app-region: drag;
 		}
diff --git a/packages/desktop/src/templates/splash.css b/packages/desktop/src/templates/splash.css
index dc76a2fe..adc2ce10 100644
--- a/packages/desktop/src/templates/splash.css
+++ b/packages/desktop/src/templates/splash.css
@@ -5,6 +5,7 @@
 	--background: oklch(1 0 0);
 	--foreground: oklch(0.145 0 0);
 	--muted-foreground: oklch(0.556 0 0);
+	--border: oklch(0.922 0 0);
 	--primary: oklch(0.205 0 0);
 	--primary-foreground: oklch(0.985 0 0);
 }
@@ -13,6 +14,7 @@
 	--background: oklch(0.145 0 0);
 	--foreground: oklch(0.985 0 0);
 	--muted-foreground: oklch(0.708 0 0);
+	--border: oklch(1 0 0 / 10%);
 	--primary: oklch(0.922 0 0);
 	--primary-foreground: oklch(0.205 0 0);
 }
@@ -39,8 +41,10 @@ body::before {
 	top: 0;
 	right: 0;
 	left: 0;
+	box-sizing: border-box;
 	/* Keep in step with the fallback in preload.ts and the variable in packages/web/src/index.css. */
 	height: 2rem;
+	border-bottom: 1px solid var(--border);
 	content: '';
 	-webkit-app-region: drag;
 }
diff --git a/packages/web/src/index.css b/packages/web/src/index.css
index ff9c5b2d..28149901 100644
--- a/packages/web/src/index.css
+++ b/packages/web/src/index.css
@@ -14,7 +14,9 @@
 	top: 0;
 	right: 0;
 	left: 0;
+	box-sizing: border-box;
 	height: var(--desktop-title-bar-height);
+	border-bottom: 1px solid var(--border);
 	content: '';
 	-webkit-app-region: drag;
 }
@@ -31,6 +33,30 @@
 	top: var(--desktop-title-bar-height);
 }
 
+/* The sidebar dividers begin below the title bar. Its bottom border meets them at a clean corner
+   instead of a vertical line running between the traffic lights. */
+.desktop-app [data-slot='sidebar-container'],
+.desktop-app [data-desktop-panel] {
+	border-right-width: 0;
+}
+
+.desktop-app [data-slot='sidebar-inner'],
+.desktop-app [data-desktop-panel] {
+	position: relative;
+}
+
+.desktop-app [data-slot='sidebar-inner']::after,
+.desktop-app [data-desktop-panel]::after {
+	position: absolute;
+	top: var(--desktop-title-bar-height);
+	right: 0;
+	bottom: 0;
+	width: 1px;
+	background: var(--sidebar-border);
+	content: '';
+	pointer-events: none;
+}
+
 /* Zoom can move the 800px desktop window into the responsive layout. Keep its floating menu and
    any left-side sheet below the traffic lights and drag surface too. */
 .desktop-app [data-desktop-floating-control] {