Size a panorama's cell and thumbnail by class: two, three or four columns

One lookup, natural_span, maps a photograph's aspect to the columns its
cell spans, and the same number names its thumbnail class, Wide2, Wide3
or Wide4, 512 pixels of long edge per column, so a 4:1 panorama is as
sharp across four columns as a frame is in one. The boundaries are
where the two neighbouring cells would leave the same share of
themselves undrawn, sqrt(s(s+1)): 2.45 and 3.46, with the first at 1.9
so a 3:2 frame stays a frame. A grid too narrow for the class falls
back to the widest that fits, the tablet gives the whole row, and a
cell asks for the class it is actually drawn at: its span, but never
more than its own class. The merge renders each wide class up to the
composite's own, which covers every fallback.

ui/dr-ui/src/merge_thumbs.rs

@@ -116,14 +116,14 @@ impl Reduced {
 }
 
 /// Which size classes a composite of this shape is thumbnailed at: the two
-/// every photograph has, and the wide one where the grid will give it a
-/// wide cell.
+/// every photograph has, and the wide classes its cell can be drawn at —
+/// its own, from the one lookup the grid packs by, and each narrower one a
+/// grid with too few columns falls back to.
 pub fn classes(width: u32, height: u32) -> Vec<ThumbSize> {
     let aspect = width as f32 / height.max(1) as f32;
+    let natural = crate::library_ui::layout::natural_span(Some(aspect));
     let mut out = vec![ThumbSize::Grid, ThumbSize::Large];
-    if crate::library_ui::layout::natural_span(Some(aspect)) > 1 {
-        out.push(ThumbSize::Wide);
-    }
+    out.extend((2..=natural).filter_map(ThumbSize::wide));
     out
 }
 
@@ -198,8 +198,14 @@ mod tests {
 
     #[test]
     fn a_panorama_is_thumbnailed_for_its_wide_cell_and_a_squat_merge_is_not() {
-        assert_eq!(classes(22_000, 5_600).last(), Some(&ThumbSize::Wide));
-        assert!(!classes(9_000, 6_000).contains(&ThumbSize::Wide));
+        use ThumbSize::*;
+        assert_eq!(
+            classes(22_000, 5_600),
+            [Grid, Large, Wide2, Wide3, Wide4],
+            "a 3.9:1 panorama spans four"
+        );
+        assert_eq!(classes(8_000, 4_000), [Grid, Large, Wide2]);
+        assert_eq!(classes(9_000, 6_000), [Grid, Large]);
     }
 
     #[test]
@@ -359,7 +365,7 @@ mod tests {
             &header,
             Some(crop),
             decoder,
-            &[ThumbSize::Grid, ThumbSize::Large, ThumbSize::Wide],
+            &classes(crop.width, crop.height),
         )
         .unwrap();