refactor(desktop): tighten the surface-var helper

Fold the null fallback into chatSurfaceRoot so both callers share one
target resolution, hoist the var names next to it, and drop the
duplicated rationale from the status-stack comment.

apps/desktop/src/app/chat/composer/status-stack/index.tsx

@@ -198,14 +198,12 @@ export function ComposerStatusStack({ queue, sessionId }: ComposerStatusStackPro
   // height never sees it. Publish our own measured height — bucketed like the
   // composer's, to avoid style invalidation churn — so the thread's
   // last-message clearance can add it and the stack never hides messages.
-  // Scoped to THIS chat surface: tiles render their own stack beside the
-  // workspace pane, and a shared global would let the taller one dictate every
-  // thread's padding (see surface-vars.ts).
+  // Scoped to THIS surface: tiles render their own stack (see surface-vars.ts).
   useLayoutEffect(() => {
     const el = stackRef.current
 
     if (!visible || !el) {
-      clearSurfaceVar(stackRef.current, STATUS_STACK_VAR)
+      clearSurfaceVar(el, STATUS_STACK_VAR)
 
       return
     }

apps/desktop/src/app/chat/surface-vars.test.ts

@@ -57,7 +57,7 @@ describe('per-surface measured-height vars', () => {
     const orphan = document.createElement('div')
     document.body.append(orphan)
 
-    expect(chatSurfaceRoot(orphan)).toBeNull()
+    expect(chatSurfaceRoot(orphan)).toBe(document.documentElement)
 
     setSurfaceVar(orphan, STATUS_STACK_VAR, '24px')
     expect(document.documentElement.style.getPropertyValue(STATUS_STACK_VAR)).toBe('24px')

apps/desktop/src/app/chat/surface-vars.ts

@@ -16,27 +16,25 @@
  * there is no single correct value to remeasure to.
  */
 
-/** Nearest enclosing chat surface, or `null` when the node is detached. */
-export function chatSurfaceRoot(el: Element | null): HTMLElement | null {
-  return el?.closest<HTMLElement>('[data-chat-surface]') ?? null
-}
-
 export const COMPOSER_HEIGHT_VAR = '--composer-measured-height'
 export const COMPOSER_SURFACE_HEIGHT_VAR = '--composer-surface-measured-height'
 export const STATUS_STACK_VAR = '--status-stack-measured-height'
 
 /**
- * Set a measured-height var on the surface owning `el`. Falls back to the
- * document root so a composer rendered outside a chat surface (the popped-out
- * window) keeps behaving exactly as before.
+ * The surface owning `el`, falling back to the document root so a composer
+ * rendered outside a chat surface (the popped-out window) keeps behaving
+ * exactly as before.
  */
+export function chatSurfaceRoot(el: Element | null): HTMLElement {
+  return el?.closest<HTMLElement>('[data-chat-surface]') ?? document.documentElement
+}
+
+/** Publish a measured-height var on the surface owning `el`. */
 export function setSurfaceVar(el: Element | null, name: string, value: string): void {
-  const target = chatSurfaceRoot(el) ?? document.documentElement
-  target.style.setProperty(name, value)
+  chatSurfaceRoot(el).style.setProperty(name, value)
 }
 
 /** Clear a measured-height var from the surface owning `el`. */
 export function clearSurfaceVar(el: Element | null, name: string): void {
-  const target = chatSurfaceRoot(el) ?? document.documentElement
-  target.style.removeProperty(name)
+  chatSurfaceRoot(el).style.removeProperty(name)
 }

apps/desktop/src/styles.css

@@ -348,8 +348,7 @@
 
     /* Space under last message vs overlay composer — driven by the measured composer height (see composer/index.tsx)
        plus the out-of-flow status stack's measured height (see status-stack/index.tsx) when one is showing.
-       Both inputs are republished per chat surface (see surface-vars.ts), and the calc is re-declared there so
-       each surface resolves against its OWN measurements — a tile's tall stack must not pad another thread. */
+       These are the defaults; each chat surface re-declares the calc against its own measurements (see below). */
     --status-stack-measured-height: 0px;
     --thread-last-message-clearance: calc(var(--composer-measured-height) + var(--status-stack-measured-height) + 2rem);