Fix macOS hang when opening Mermaid plans

[warp-agent-staging]

Description

Fixes APP-5824 by initializing rendered Mermaid offsets when a lazy plan editor binds to its window, before its first element layout. Lazy edit flushing on macOS now lays out sequentially, avoiding a main-thread wait on Rayon workers concurrently entering CoreText.

Linked Issue

• APP-5824
• Screenshots or video included — exact-path capture remained blocked after a second macOS retry because the available credential is a service account, not a GUI user.

Testing

• The lazy regression now flushes first layout and asserts a positive-size MermaidDiagram at the command range; the pre-fix form fails on master. The sequential regression compares wrapped text, multiline code, hidden-block placement, and rendered frame/line geometry.
• cargo nextest run -p warp_editor (502 passed)
• cargo nextest run -p warp -E 'test(notebooks::editor::model::tests)' (72 passed)
• cargo clippy --workspace --exclude warp_completer --all-targets --tests -- -D warnings
• cargo clippy -p warp --all-targets --tests -- -D warnings
• cargo clippy -p warp_completer --all-targets --tests -- -D warnings
• ./script/format --check, ./script/check_no_inline_test_modules, and cargo check -p warp_editor -p warp --all-targets --tests
• macOS 26.3.1 arm64: both strengthened focused regressions and cargo build --bin warp passed at 0005ccf7243e736d2b628fc780d269ed88b96043.
• I have manually tested my changes locally with ./script/run — a second interactive macOS retry prevented display sleep and launched Warp, but the injected credential is a service account (Unauthorized: Expected a user account), leaving the GUI in onboarding and the exact conversation redirected to login. The tab-switch and replacement process sample remain blocked; no unrelated proxy was substituted.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-BUG-FIX: Fixed a hang when opening restored plans containing Mermaid diagrams on macOS.

[warp-agent-staging]

This PR was generated with Warp.

Comment @warp-staging-factory on this PR to send it follow-up work.

  

[warp-agent-staging]

Overview

The code, regression coverage, and lifecycle contract findings are resolved. One acceptance-verification gap requires human judgment.

Concerns

• No recording or process sample proves the opted-in macOS tab-switch path. Both attempts were blocked by runner authentication rather than product behavior; verify with an authorized macOS user/test fixture and capture the switch plus a sample showing Presenter::layout no longer spends most of its time in Rayon LockLatch::wait_and_reset.

Verdict

Checks: build pass, tests pass, CI draft-skipped, visual proof missing

Found: 0 critical, 1 important, 0 suggestions, 0 nits