On a 32-bit Amlogic TV box, 57% of UI frames while moving focus along a rail
missed the 16.68 ms budget, and the median frame was already over it at 20.7 ms.
The GPU was idle throughout (raster p90 9.1 ms, zero frames over budget) -- all
of it was main-isolate work, split roughly evenly between layout and semantics.
Five separate causes, all measured:
- The horizontal rail used `itemExtentBuilder`, so `RenderSliverVariedExtentList`
walked every preceding index on each realized child, in every layout pass, and
re-resolved the trailing slot inside the closure. Pinning the trailing cell to
the card extent makes the list uniform, which restores O(1) offset, index and
max-extent math.
- The sidebar-expand tween wrapped the whole shell with a `LayoutBuilder` inside
its builder, so ~15 ticks per focus flip rebuilt `SideNavigationRail` -- and its
non-virtualized child list -- during the layout phase. The tween now wraps only
the content `Positioned`, matching `SideNavigationBleedBuilder`.
- Each nav item crossfaded through two `Opacity` subtrees, i.e. 14-18 offscreen
save layers per frame for the whole 250 ms morph. It now fades colour alpha on
the leaf, the same substitution `AnimatedDimScrim` already documents.
- `HorizontalScrollWithArrows` kept a scroll listener on platforms where its arrow
chrome is compiled out, so crossing either scroll boundary `setState`d a whole
rail row of cards. Two full-row rebuild storms per traversal, for arrows that
cannot appear.
- The rail's semantic proxy rebuilt its label and six closures on every
`_RailFocusModel` notification, including vertical-scroll flips.
Also: one merged listener per `SettingsBuilder` instead of six, and the media card
semantic label is cached rather than rebuilt and then discarded by the rail's
`ExcludeSemantics`.
Measured on the device, median of three runs: frames over budget 120/212 -> 32/246,
total UI-thread work 4067 ms -> 1845 ms, layout p90 18.2 -> 8.1 ms, build p90
7.95 -> 2.42 ms. The sidebar scenario went from 121/148 frames over budget to
32/184.
Memoize the per-image SHA-1 disk-cache key (ran on the UI thread on
every image build), fade images in via paint alpha instead of an
AnimatedSwitcher saveLayer per in-flight tile, create SettingsBuilder's
merged listenable once instead of per build, and drop the pointer
chrome (InkWell ink machinery, cursor MouseRegions) from cards on TV
where no pointer exists.
edde746