Histogram: scale bar heights linearly to the tallest bin, not sorted.get(250) (#6638)

crates/ui-egui/src/tone.rs

@@ -217,11 +217,17 @@ pub fn keep_after_commit(app: &mut PhotocraftApp, layer: LayerId, before: Option
     }
 }
 
+/// Display scale for a 256-bin histogram: bar heights are `v / top` clamped to 1, linear
+/// to the tallest bin like Photoshop. A percentile of the sorted bins degenerates on
+/// sparse histograms (few occupied levels sort 0 into every low index), drawing tiny
+/// counts as tall as the real peaks (#6574).
+/// Extracted from [`draw_histogram`] so the scaling is unit-testable.
+pub fn histogram_scale_top(h: &[u32; 256]) -> f32 {
+    h.iter().copied().max().unwrap_or(1).max(1) as f32 * 1.1
+}
+
 pub fn draw_histogram(p: &egui::Painter, r: Rect, h: &[u32; 256], color: Color32) {
-    // Scale to a high percentile so a single spike (e.g. pure white) doesn't flatten the rest.
-    let mut sorted: Vec<u32> = h.to_vec();
-    sorted.sort_unstable();
-    let top = sorted.get(250).copied().unwrap_or(1).max(1) as f32 * 1.1;
+    let top = histogram_scale_top(h);
     let mut pts = vec![egui::pos2(r.left(), r.bottom())];
     for (i, v) in h.iter().enumerate() {
         let x = r.left() + (i as f32 + 0.5) / 256.0 * r.width();
@@ -452,6 +458,34 @@ mod tests {
         );
         assert_ne!(tag(HistSource::Channel { layer, surface: GraySurface::LayerMask }, ToneSpace::Rgb), tag(HistSource::Layer(layer), ToneSpace::Rgb));
     }
+
+    /// #6574: bar heights scale linearly to the tallest bin. A 90/10 black/white
+    /// histogram draws the small bar at a ninth of the big one, not full height.
+    #[test]
+    fn histogram_scale_is_proportional_for_two_level_images() {
+        let mut h = [0u32; 256];
+        h[0] = 100_000;
+        h[255] = 900_000;
+        let top = histogram_scale_top(&h);
+        let small = (h[0] as f32 / top).min(1.0);
+        let big = (h[255] as f32 / top).min(1.0);
+        assert!((small / big - 1.0 / 9.0).abs() < 0.02, "small/big = {}, top = {top}", small / big);
+    }
+
+    /// #6574: a sparse histogram (few occupied levels) must not scale to a bin
+    /// index that is 0: the old `sorted.get(250)` degenerated to `top = 1.1`,
+    /// drawing a few hundred samples as tall as hundreds of thousands.
+    #[test]
+    fn histogram_scale_does_not_degenerate_on_sparse_histograms() {
+        let mut h = [0u32; 256];
+        h[0] = 500_000;
+        h[128] = 300;
+        h[255] = 500_000;
+        let top = histogram_scale_top(&h);
+        assert!(top > h[128] as f32, "scale {top} must exceed the tiny middle bin");
+        let tiny = (h[128] as f32 / top).min(1.0);
+        assert!(tiny < 0.05, "tiny bin draws near-flat, got {tiny}");
+    }
 }
 
 #[cfg(test)]