perf(ui): cache-rendering-hint on static shell chrome (#617)

cache-rendering-hint: true on HeaderBar, NowPlayingBar and Sidebar roots so
femtovg caches those subtrees to a persistent texture instead of
re-tessellating them on every repaint. Sidebar root is safe: the Large-NPB
spectrum dock is an AppShell overlay, not a Sidebar child. SMOKE-WATCH:
playlists-list scroll (Flickable-inside-cache); narrow to nav-only if it
regresses.

Authored by Claude (cc) in the repaint-cache-hint worktree; committed
as-is by km so both #617 lines share one canonical commit.

km

crates/qbz-ui/ui/shell/HeaderBar.slint

@@ -548,6 +548,15 @@ export component HeaderBar inherits Rectangle {
     // `header-menu-navigate` route handler the dropdown menus use.
     callback navigate(string /* route */);
 
+    // #617 per-frame cost: femtovg has no partial rendering, so the animated
+    // now-playing equalizer bars repaint the WHOLE window ~8x/sec. This header is
+    // static at idle (no internal scroll; the search cortinilla is a separate
+    // AppShell overlay), so cache its subtree to a persistent texture: every
+    // repaint then skips the header's tree-walk + path re-tessellation and blits
+    // the cache instead. Invalidates only when something inside changes (typing,
+    // hover, resize) — not per frame. See spec 2026-07-19-cpu-idle-repaint-617 §9.2.
+    cache-rendering-hint: true;
+
     height: Layout.header-height;
     background: ShellState.app-background-active
         ? Theme.surface-card.with-alpha(AppearanceState.app-background-surface-alpha)

crates/qbz-ui/ui/shell/NowPlayingBar.slint

@@ -19,6 +19,17 @@ export component NowPlayingBar inherits Rectangle {
     callback media-action(string, string, string);
     callback settings-bool(string, bool);
 
+    // #617 per-frame cost: the always-visible bottom bar is a high-item-count
+    // subtree (SongCard cover, seek line, transport controls, quality badges,
+    // volume) that is static except for the ~1 Hz seek push (verified: SeekBar's
+    // only animates are hover-opacity one-shots; progress = whole-second position).
+    // Caching it drops the bar's tree-walk + path re-tessellation from ~8x/sec
+    // (driven by the eq-bar full-window repaints) to ~1x/sec; the other 7 frames
+    // blit the cached texture. See spec 2026-07-19-cpu-idle-repaint-617 §9.2.
+    // NOTE: Large mode's sidebar spectrum dock is NOT inside this component; it is
+    // in Sidebar and must stay un-cached (30 fps visualizer).
+    cache-rendering-hint: true;
+
     // EXPLICIT mode-aware height on the wrapper itself. The bottom bar lives in
     // AppShell's outer VerticalLayout slot, which sizes a plain Rectangle child
     // to its CONTENT height — and a Rectangle does NOT auto-adopt a conditional

crates/qbz-ui/ui/shell/Sidebar.slint

@@ -693,6 +693,21 @@ export component Sidebar inherits Rectangle {
     clip: true;
     animate width { duration: 160ms; easing: ease-in-out; }
 
+    // #617 per-frame cost: cache this whole subtree to a persistent texture so
+    // the eq-bar's ~8x/sec full-window repaints skip the sidebar's tree-walk +
+    // path re-tessellation (nav rows + playlists list) and blit the cache instead.
+    // SAFE to cache the root here: the animated Large-NPB spectrum dock is NOT a
+    // child of the Sidebar — it is an AppShell-root overlay (see the dock-band
+    // RESERVATION comment ~line 1120); everything actually inside the Sidebar is
+    // static at idle (the scrollbar thumb only animates on hover; the sort/context
+    // menus are separate PopupWindow surfaces, not part of this texture).
+    // Invalidates on: playlist scroll (transient), width toggle (160ms transient),
+    // hover, list changes. SMOKE-WATCH: scroll the playlists list and confirm no
+    // clipping/offset artifact from the Flickable-inside-cache interaction; if it
+    // regresses, narrow the hint to the static nav section only.
+    // See spec 2026-07-19-cpu-idle-repaint-617 §9.2.
+    cache-rendering-hint: true;
+
     VerticalLayout {
         // Per-side padding so Large can zero ONLY the bottom: the reserved dock
         // band (and the AppShell overlay aligned to it) then reaches the true