Work in progress found uncommitted in the tree, committed as its own
change so the fixes that follow can be read separately. Not authored in
this session; the description below is written from the diff.
`load_window` rebuilds every row, and a scroll reloads once the view has
travelled a quarter of the loaded window — so three quarters of the cells
being rebuilt are the same photographs already on screen. Rebuilding them
empty blanked the grid to `Theme.ground` and refilled it a beat later,
once a worker had re-read and re-decoded each one from the store. That is
the black flash on every screenful of scrolling, and on a column change,
a zoom step, a filter and a return from develop.
Thumbnails are now held by `image_id` across the swap — a refcount per
cell, no pixels move — along with the "no preview" verdict, which is an
answer about the file worth keeping for the same reason. `requested` is
rebuilt from what the new model actually holds rather than cleared, so a
carried cell is not fetched again while one newly scrolled in still is.
The size class each cell's pixels came from is tracked alongside, so a
grid zoomed past that class still asks for the sharper one.
Also guards the whole `scrolled` and `columns-changed` handlers on
`show-library` rather than just the resume latch: a Flickable being torn
down passes its viewport through zero, which was indistinguishable from a
fling to the top and reloaded the window against the first rows of the
catalog every time an image was opened.
dtourolle
deabac0923