diff --git a/tools/ui/src/lib/components/app/dialogs/DialogMermaidPreview.svelte b/tools/ui/src/lib/components/app/dialogs/DialogMermaidPreview.svelte
index 09e53442ac65..e741373497aa 100644
--- a/tools/ui/src/lib/components/app/dialogs/DialogMermaidPreview.svelte
+++ b/tools/ui/src/lib/components/app/dialogs/DialogMermaidPreview.svelte
@@ -14,6 +14,7 @@
 <Dialog.Root bind:open {onOpenChange}>
 	<Dialog.Content
 		class="z-999999 grid max-h-full max-w-full! grid-rows-[1fr_auto] overflow-hidden p-0 md:h-[90vh] md:max-w-[90vw]!"
+		showCloseButton
 	>
 		<MermaidPreview {svgHtml} />
 	</Dialog.Content>
diff --git a/tools/ui/src/lib/components/ui/dialog/dialog-close.svelte b/tools/ui/src/lib/components/ui/dialog/dialog-close.svelte
index e8a96a773163..cd957a9bf8d3 100644
--- a/tools/ui/src/lib/components/ui/dialog/dialog-close.svelte
+++ b/tools/ui/src/lib/components/ui/dialog/dialog-close.svelte
@@ -5,3 +5,9 @@
 </script>
 
 <DialogPrimitive.Close bind:ref data-slot="dialog-close" {...restProps} />
+
+<style>
+	:global([data-dialog-close]) {
+		z-index: 999;
+	}
+</style>
diff --git a/tools/ui/src/lib/components/ui/dialog/dialog-content.svelte b/tools/ui/src/lib/components/ui/dialog/dialog-content.svelte
index e6b2a641d5d2..8a1a5909d579 100644
--- a/tools/ui/src/lib/components/ui/dialog/dialog-content.svelte
+++ b/tools/ui/src/lib/components/ui/dialog/dialog-content.svelte
@@ -10,7 +10,7 @@
 		class: className,
 		portalProps,
 		ref = $bindable(null),
-		showCloseButton = true,
+		showCloseButton = false,
 		...restProps
 	}: WithoutChildrenOrChild<DialogPrimitive.ContentProps> & {
 		portalProps?: DialogPrimitive.PortalProps;
@@ -25,7 +25,8 @@
 	<DialogPrimitive.Content
 		bind:ref
 		class={cn(
-			`fixed top-[50%] left-[50%] z-50 grid max-h-[100dvh] w-full max-w-[calc(100%-2rem)] translate-x-[-50%] translate-y-[-50%] gap-4 overflow-y-auto rounded-lg border border-border/30 bg-background p-6 shadow-lg duration-200 data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=closed]:fill-mode-forwards data-[state=closed]:zoom-out-95 data-[state=open]:animate-in data-[state=open]:fade-in-0 data-[state=open]:zoom-in-95 sm:max-w-lg md:max-h-[100vh]`,
+			`fixed top-[50%] left-[50%] z-50 grid translate-x-[-50%] translate-y-[-50%] gap-4 overflow-y-auto rounded-lg border border-border/30 bg-background p-6 shadow-lg duration-200 data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=closed]:fill-mode-forwards data-[state=closed]:zoom-out-95 data-[state=open]:animate-in data-[state=open]:fade-in-0 data-[state=open]:zoom-in-95`,
+			'max-h-[100dvh] max-w-full size-auto sm:max-w-lg md:w-auto md:size-auto md:max-w-[calc(100%-2rem)]',
 			className
 		)}
 		data-slot="dialog-content"
diff --git a/tools/ui/src/lib/components/ui/dialog/dialog-header.svelte b/tools/ui/src/lib/components/ui/dialog/dialog-header.svelte
index f56269414558..fd590e186044 100644
--- a/tools/ui/src/lib/components/ui/dialog/dialog-header.svelte
+++ b/tools/ui/src/lib/components/ui/dialog/dialog-header.svelte
@@ -1,20 +1,43 @@
 <script lang="ts">
+	import XIcon from '@lucide/svelte/icons/x';
 	import { cn, type WithElementRef } from '$lib/components/ui/utils';
+	import { Dialog as DialogPrimitive } from 'bits-ui';
 	import type { HTMLAttributes } from 'svelte/elements';
 
 	let {
 		children,
 		class: className,
 		ref = $bindable(null),
+		showCloseButton = true,
 		...restProps
-	}: WithElementRef<HTMLAttributes<HTMLDivElement>> = $props();
+	}: WithElementRef<HTMLAttributes<HTMLDivElement>> & {
+		showCloseButton?: boolean;
+	} = $props();
 </script>
 
+<!--
+	Header is `sticky`, so it stays at the top while the dialog body scrolls. The close
+	button lives here (not in the body) so it sticks together with the title. `sticky`
+	makes it the containing block, so the close can be absolutely placed at its corner.
+-->
 <div
 	bind:this={ref}
-	class={cn('flex flex-col gap-2 text-center sm:text-left', className)}
+	class={cn(
+		'flex flex-col gap-2 text-center sm:text-left sticky top-0 z-50 bg-background md:bg-transparent',
+		className
+	)}
 	data-slot="dialog-header"
 	{...restProps}
 >
 	{@render children?.()}
+
+	{#if showCloseButton}
+		<DialogPrimitive.Close
+			class="absolute top-0 right-0 rounded-xs opacity-70 ring-offset-background transition-opacity hover:opacity-100 focus:ring-2 focus:ring-ring focus:ring-offset-2 focus:outline-hidden disabled:pointer-events-none [&_svg]:pointer-events-none [&_svg]:shrink-0 [&_svg:not([class*='size-'])]:size-4"
+		>
+			<XIcon />
+
+			<span class="sr-only">Close</span>
+		</DialogPrimitive.Close>
+	{/if}
 </div>