Linear: CODAGT-313

Summary

When many messages are queued in the agent chat, the chat history becomes unscrollable: mouse wheel and scrollbar drag both stop responding.

The input wrapper in AgentChatPageView.tsx:496 is shrink-0 overflow-y-auto with no max-height, so overflow-y-auto is a no-op and the section grows unbounded as QueuedMessagesList adds rows. Its sibling ChatScrollContainer is flex-1 min-h-0, so it absorbs the shrinkage and clientHeight collapses to 0. The chat list is then a zero-height viewport with nothing to scroll.

Measured against the actual AgentChatPageView rendered in Storybook with 20 queued messages (1280x800):

Demo

Left (main): wheel-up does nothing because the chat scroll container has been crushed to zero height.
Right (this PR): the queued list scrolls inside its own pane and the chat history scrolls normally.

Recording is AgentChatPageView rendered through Storybook with the production component source. The same gesture (wheel-up over the chat history, then wheel-down over the queued list) is applied to both sides. Source for the recording is in bpmct/codagt-313-assets.

Change

-		<div className={cn("flex w-full flex-col", className)}>
+		// Cap the queue at ~40% of the small viewport so a long queue
+		// does not push the chat history's scroll container down to
+		// zero height (CODAGT-313). The list scrolls inside its own pane.
+		<div
+			className={cn(
+				"flex w-full flex-col max-h-[40svh] overflow-y-auto [scrollbar-gutter:stable] [scrollbar-width:thin] [scrollbar-color:hsl(var(--surface-quaternary))_transparent]",
+				className,
+			)}
+		>

Why this spot, not the outer wrapper

The composer textarea already self-caps at max-h-[50vh] in ChatMessageInput.tsx:688, so the only unbounded growth source in the input section is the queued list. Capping the list keeps the constraint colocated with the component that owns it, and any future consumer of QueuedMessagesList is automatically safe.

40svh (small viewport height) so the queue doesn't fight with the iOS keyboard once it appears, matching the h-dvh decision in #24848.

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