When handling null/undefined, be explicit about intent:

• Choose the correct fallback source: if a variable is undefined, ensure ?? falls back to the intended “truth” (not a stale previous value).
• Avoid silent dropping: don’t filter out non-null ReactNode variants in a way that changes semantics between render modes (inline vs overflow). If separators/text are part of the host contract, preserve them intentionally.
• Prefer exhaustiveness over implicit null: for union/enum switches, use an exhaustive pattern so new members don’t silently fall through to null.
• Match tests to the real nullability contract: if a value is always a boolean, assert false (not undefined).

Example (routing fallback):

const reportedWorkspaceCwd = connection.sessionId
  ? connection.workspaceCwd
  : (selectedWorkspaceCwd ?? primaryWorkspaceCwd);
// If selectedWorkspaceCwd is undefined, fall back to primaryWorkspaceCwd—not a stale connection.workspaceCwd.

Example (exhaustive switch):

switch (checks) {
  case 'passing': return ...;
  case 'failing': return ...;
  case 'none': return null;
  default: {
    const _exhaustive: never = checks;
    return null;
  }
}

Example (test nullability):

expect(latestChatEditorProps.disabled).toBe(false); // not undefined