A session with 1935 persisted queue rows came up frozen for ~28s at
120% CPU, every launch, deterministically. It looked like corrupted
local state — moving ~/.local/share/qbz aside "fixed" it — but nothing
was corrupt: that is simply a queue the UI could not carry.
The chain. AppShell mounts ImmersiveView unconditionally (its own
comment: while closed it is "an invisible, non-interactive Item"), and
`visible: false` stops an item painting, never building. CoverflowPanel
reads the flat WHOLE-queue document — unlike the queue panel's, which
paginates at 40 — and runs two Repeaters over it, the forward list and
a reversed copy. So every launch built ~4000 CoverCards, each an image
with a masked layer, for a panel nobody had opened.
Two gates, because either alone leaves it broken:
- CoverflowPanel and both QueueTabsPanel mounts move behind a Loader
keyed on the condition they already carried in `visible:`. That is
the launch freeze.
- The coverflow delegates become zero-size, non-clipping slots whose
CoverCard exists only within |offset| < 6.5 (the fan paints 3.5;
the margin covers the 320ms ease). That is opening it, which the
Loader alone would still freeze.
The reference does create one element per row and hides |offset| > 3
(CoverflowPanel.slint), and that is fine there: a Slint element is
cheap where a QML delegate is not. Copying its shape literally is what
cost the 28 seconds.
Verified against the real session state, queue restored, not a
synthetic one: 120% CPU -> 2.9%, and a screenshot of Discover fully
painted where it used to sit on the skeleton.
cc
vicrodh
66ba12bd53