fix(max): constrain AI chat composer height

[lordspline]

Problem

PostHog AI chat composer can grow too tall when a long prompt is pasted, which can leave the send and queue controls unreachable in constrained viewports.

Fixes PostHog#62461

Changes

• Cap the Max composer content to the available dynamic viewport.
• Let queued messages scroll instead of pushing the input controls out of reach.
• Cap the chat textarea with Max-specific styling so it still auto-grows below the cap and scrolls internally above it.
• Add component coverage for long pasted content, Enter vs Shift+Enter submission behavior, and disabled/loading controls.

How did you test this code?

Agent-authored verification:

• pnpm --filter=@posthog/frontend format
• hogli test frontend/src/scenes/max/components/QuestionInput.test.tsx
• pnpm --filter=@posthog/frontend typegen:write to generate ignored local Kea type files before type checking
• pnpm --filter=@posthog/frontend typescript:check
• Browser verification against local PostHog: pasted a 100-line prompt in the main scene, thread view on a laptop viewport, thread view on a narrow viewport, side panel on desktop and narrow viewports, and an extra very-short viewport; confirmed the textarea scrolls internally and the send control remains visible/reachable. Also recorded a paste, internal-scroll, and send flow.

👉 Stay up-to-date with PostHog coding conventions for a smoother review.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

Docs update

Not needed for this UI bug fix.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Captain Capy implemented this from a human-directed bug report. I reproduced and measured the composer in the main scene, side panel, laptop, narrow, and very-short viewport cases before changing the layout.

I chose a Max-specific viewport cap rather than changing the shared LemonTextArea API, after checking the queued-message block, slash-command autocomplete, and hands-free surface. The goal is to preserve existing textarea autosize, keyboard behavior, focus, labeling, shared-thread behavior, and autocomplete anchoring while preventing the composer from consuming the viewport.

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