diff --git a/frontend/src/lib/components/common/modal/Modal.svelte b/frontend/src/lib/components/common/modal/Modal.svelte
index d718c3c1bd..20821a33f9 100644
--- a/frontend/src/lib/components/common/modal/Modal.svelte
+++ b/frontend/src/lib/components/common/modal/Modal.svelte
@@ -20,6 +20,11 @@
 		style?: string
 		cancelText?: string | undefined
 		kind?: 'button' | 'X'
+		/** Make the dialog fill the height it is anchored to and lay its body out as a flex
+		 * column, so content can size itself with `h-full` / `flex-1 min-h-0`. Off by default:
+		 * the dialog otherwise hugs its content, and percentage heights inside it do not
+		 * resolve (the centering wrapper is `min-h-full`, i.e. height:auto). */
+		fillHeight?: boolean
 		/** Force a minimum z-index base. Defaults to elevating above the AI chat
 		 * side panel when it is open. Pass an explicit value to stack above other
 		 * surfaces (e.g. a modal opened over the /sessions preview-pane editor). */
@@ -36,6 +41,7 @@
 		style = '',
 		cancelText = undefined,
 		kind = 'button',
+		fillHeight = false,
 		minZIndex: minZIndexProp = undefined,
 		settings,
 		children: children_render,
@@ -119,12 +125,17 @@
 					></div>
 
 					<div class="{posClass} inset-0 z-10 overflow-y-auto">
-						<div class="flex min-h-full items-center justify-center p-4">
+						<div
+							class="flex {fillHeight ? 'h-full' : 'min-h-full'} items-center justify-center p-4"
+						>
 							<!-- svelte-ignore a11y_no_static_element_interactions -->
 							<div
 								onclick={stopPropagation(bubble('click'))}
 								class={twMerge(
-									'relative transform overflow-hidden rounded-md bg-surface px-4 pt-5 pb-4 text-left shadow-xl transition-all sm:my-8 sm:w-full sm:max-w-lg sm:p-6',
+									'relative transform overflow-hidden rounded-md bg-surface px-4 pt-5 pb-4 text-left shadow-xl transition-all sm:w-full sm:max-w-lg sm:p-6',
+									// The margins are what keeps a content-sized dialog off the viewport edges; a
+									// filling one takes its inset from the wrapper's padding instead.
+									fillHeight ? 'h-full flex flex-col' : 'sm:my-8',
 									c,
 									open
 										? 'ease-out duration-300 opacity-100 translate-y-0 sm:scale-100'
@@ -137,16 +148,16 @@
 										><CloseButton on:close={() => (open = false)} /></div
 									>
 								{/if}
-								<div class="flex">
+								<div class="flex {fillHeight ? 'flex-1 min-h-0' : ''}">
 									<!-- min-w-0: without it this flex item takes its content's min-content width and
 									     stretches the modal past its max-width instead of letting content shrink. -->
-									<div class="text-left flex-1 min-w-0">
+									<div class="text-left flex-1 min-w-0 {fillHeight ? 'flex flex-col min-h-0' : ''}">
 										<div class="flex flex-row items-center justify-between">
 											<h3 class="text-emphasis text-lg font-semibold">{title}</h3>
 											{@render settings?.()}
 										</div>
 
-										<div class="mt-4 text-sm text-primary">
+										<div class="mt-4 text-sm text-primary {fillHeight ? 'flex-1 min-h-0' : ''}">
 											{@render children_render?.()}
 										</div>
 									</div>
diff --git a/frontend/src/lib/components/copilot/chat/script/MermaidDisplay.svelte b/frontend/src/lib/components/copilot/chat/script/MermaidDisplay.svelte
index dda1892baa..60687efcce 100644
--- a/frontend/src/lib/components/copilot/chat/script/MermaidDisplay.svelte
+++ b/frontend/src/lib/components/copilot/chat/script/MermaidDisplay.svelte
@@ -35,6 +35,13 @@
 				startOnLoad: false,
 				theme: dark ? 'dark' : 'default',
 				securityLevel: 'strict',
+				// Mermaid writes this stack into a <style> block inside the SVG, so the app's font
+				// never reaches a diagram: the bundled vector emoji font has to be named here or
+				// emoji fall back to the platform bitmap one, which ignores the zoom transform
+				// (app.css). Inter sits last before it because the emoji font also covers digits,
+				// # and *; on a box without the MS core fonts those would otherwise render as its
+				// keycap glyphs. Keeping Inter behind them leaves diagram typography unchanged.
+				fontFamily: '"trebuchet ms", verdana, arial, Inter, "Noto Color Emoji", sans-serif',
 				// Throw on parse errors instead of injecting an orphan error diagram into the DOM.
 				suppressErrorRendering: true
 			})
@@ -43,6 +50,17 @@
 			if (seq !== renderSeq) return
 			svg = result.svg
 			renderedCode = source
+			// Mermaid sizes each node by measuring its label, so a font still in flight yields
+			// boxes cut to fallback metrics that the real glyphs then overflow. Drawing the
+			// diagram is itself what asks for the fonts its glyphs need — including the emoji
+			// subsets — so let them settle and lay it out again against the true metrics.
+			if (document.fonts?.status === 'loading') {
+				await document.fonts.ready
+				if (seq !== renderSeq) return
+				const settled = await mermaid.render(`mermaid-${randomUUID()}`, source)
+				if (seq !== renderSeq) return
+				svg = settled.svg
+			}
 		} catch {
 			// Parse failure (often a partial block still streaming in): fall back to the
 			// raw source. `showSvg` already hides any previous diagram since `renderedCode`
@@ -145,7 +163,7 @@
 		</div>
 	</div>
 
-	<Modal bind:open={expanded} title="Diagram" kind="X" class="sm:max-w-none w-[92vw]">
+	<Modal bind:open={expanded} title="Diagram" kind="X" fillHeight class="sm:max-w-none w-[92vw]">
 		{#snippet settings()}
 			<div class="flex flex-row gap-1 mr-8">
 				<Button
@@ -183,7 +201,7 @@
 			</div>
 		{/snippet}
 		<div
-			class="relative w-full h-[78vh] overflow-hidden rounded border cursor-grab bg-surface-secondary"
+			class="relative w-full h-full overflow-hidden rounded border cursor-grab bg-surface-secondary"
 		>
 			{#if expanded}
 				<div use:panzoomAction class="w-full h-full flex items-center justify-center">
diff --git a/frontend/src/lib/components/sessions/PreviewTabHost.svelte b/frontend/src/lib/components/sessions/PreviewTabHost.svelte
index 8cd91b456f..4ec8ec9601 100644
--- a/frontend/src/lib/components/sessions/PreviewTabHost.svelte
+++ b/frontend/src/lib/components/sessions/PreviewTabHost.svelte
@@ -127,14 +127,16 @@
 		active ? 'z-10 opacity-100 pointer-events-auto' : 'z-0 opacity-0 pointer-events-none'
 	)
 
-	// Overlays the editor opens (drawers, modals, popovers) anchor here rather than to the
+	// Overlays a tab opens (drawers, modals, popovers) anchor here rather than to the
 	// document, so they stay within this tab and hide with it when another tab takes over.
+	// Every branch that renders content in-realm must bind this — an unbound host makes the
+	// overlay fall back to viewport-`fixed`, spilling it across the whole app.
 	// The stack is per-tab for the same reason: this host stays mounted while hidden, and a
 	// shared stack would let its overlays arbitrate Escape for the tab the user is looking at.
-	let editorEl: HTMLDivElement | undefined = $state()
+	let overlayHostEl: HTMLDivElement | undefined = $state()
 	let hostDrawers = $state({ val: [] as string[] })
 	setOverlayHost({
-		el: () => editorEl,
+		el: () => overlayHostEl,
 		drawers: hostDrawers,
 		// The panel is resized rather than unmounted, so the active tab of a panel that
 		// is off screen is as invisible as a background tab.
@@ -180,7 +182,7 @@
 
 {#if slot.kind === 'editor' && mounted && runtime}
 	<div
-		bind:this={editorEl}
+		bind:this={overlayHostEl}
 		class="absolute inset-0 flex flex-col min-h-0 bg-surface {visibility}"
 		aria-hidden={!active}
 	>
@@ -238,7 +240,11 @@
 		{/if}
 	</div>
 {:else if slot.kind === 'artifact' && mounted}
-	<div class="absolute inset-0 flex flex-col min-h-0 bg-surface {visibility}" aria-hidden={!active}>
+	<div
+		bind:this={overlayHostEl}
+		class="absolute inset-0 flex flex-col min-h-0 bg-surface {visibility}"
+		aria-hidden={!active}
+	>
 		{#if artifact}
 			<ArtifactViewer {artifact} />
 		{:else if !runtime?.manager.artifacts.loading}