fix(wallpaper): skip ImageHandle::from_rgba when source and fit unchanged

[rchennau]

Problem

cosmic-settings desktop leaks ~9 GB/hour of RSS when running as a persistent background daemon with slideshow mode active. The process is eventually OOM-killed after exhausting swap, which starves the Wayland compositor of CPU time and freezes the display. This is a reproduction of #843.

Root Cause

cache_display_image() calls ImageHandle::from_rgba() on every Message::UpdateState event (fired once per wallpaper rotation by the inotify watcher on the cosmic-bg state file). from_rgba unconditionally calls Id::unique():

// pop-os/iced core/src/image.rs:216
static NEXT_ID: AtomicU64 = AtomicU64::new(0);
Self(_Id::Unique(NEXT_ID.fetch_add(1, atomic::Ordering::Relaxed)))

Every call produces a monotonically-increasing unique ID, making cache hits in the wgpu raster cache (wgpu/src/image/raster.rs — FxHashMap<image::Id, Memory>) structurally impossible.

Eviction only happens inside trim() (raster.rs:78), which is only called from draw() (wgpu/src/lib.rs:162), which is only called from present(). When the Wayland compositor suppresses RedrawRequested between rotations (DPMS off, occlusion, throttling), present() never fires, trim() never runs, and the FxHashMap grows unbounded — one Memory entry per rotation interval.

This is the same trim()/present() gap described in iced-rs/iced#2659, triggered here by the slideshow rotation path.

Fix

Add cached_display_key: Option<DefaultKey> and cached_display_fit: Option<usize> to Page. At the start of cache_display_image(), resolve the effective DefaultKey for the current selection and short-circuit if it matches the previously rendered values:

if self.cached_display_handle.is_some()
    && resolved_key == self.cached_display_key
    && Some(self.selected_fit) == self.cached_display_fit
{
    return;  // source unchanged — existing handle is valid
}

ImageHandle::from_rgba is now called only when the wallpaper or fit mode actually changes, not on every state event. All three existing cached_display_handle = None sites are updated to clear the new fields so state stays coherent.

Tested

• Pop!_OS 24.04, cosmic-settings v1.0.12, NVIDIA RTX 5060 Ti + Intel iGPU (hybrid display path)
• Built from source, deployed, and monitored for 20 minutes
• RSS stable at ~243 MB (previously grew at ~9.3 GB/hour)
• Slideshow rotation continues to update the display preview correctly

Note on complementary fix

This prevents new unique handles from accumulating. A complementary fix in pop-os/iced — decoupling trim() from present() so stale entries are evicted even when the compositor suppresses redraws — would provide defence-in-depth (see iced-rs/iced#2659).

[leviport]

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