`cell-size`, `columns` and `visible-rows` were all derived from the
LibraryGrid's own width and height. The cells are not drawn in that box:
the capture-time axis is a sibling of the grid, 96px of it, and the
header takes another 44px off the top.
So `columns` counted the timeline as room for thumbnails and fitted one
more column than there was space for. The last column started inside the
Flickable and ended outside it — clipped, with no sideways scroll to
reach it. At the 180px size class on a phone in portrait the timeline is
a quarter of the screen, which is most of a column.
`visible-rows` was wrong the same way and fed `capacity`, so every window
over-fetched by the ratio of the header to the viewport.
Both now read `grid-area`, the layout the cells actually live in. That is
a descendant, and reading a descendant's geometry is the shape that
causes binding loops elsewhere in this UI — but not here: nothing derived
from these feeds back into the layout. The cells are placed absolutely
inside the Flickable and a Flickable's layout constraints are a bare
`stretch: 1` that its viewport cannot influence.