[arnestrickmann]

Summary

• Paint the macOS hidden title-bar drag surface with the page background so the sidebar divider cannot show through it.
• Add a regression test for the opaque title-bar surface.
• Keep the chat-header gap fix out of this PR; that fix is already on main via keep Chat's header right under the desktop title bar #207.

Verification

• Web tests: 324 passed, 0 failed.
• Focused desktop tests: 3 passed, 0 failed.
• Web typecheck: passed.
• Web lint: passed with one existing unrelated only-export-components warning.
• Web production build: passed.
• Full repository test run reaches one pre-existing sandbox backend HOME assertion failure; this PR does not modify that package.
• Independent adversarial reviews: no findings.

Scope

Two files: one CSS background declaration with updated comments, and one regression test. No documentation or prompt changes are needed.