Merge branch 'histogram'

# Conflicts:
#	ui/dr-ui/src/lib.rs

ui/dr-ui/src/lib.rs

@@ -19,6 +19,7 @@ mod collections_ui;
 mod derived_sync;
 mod develop;
 mod export;
+mod histogram;
 mod labels;
 mod library;
 mod library_ui;
@@ -261,6 +262,11 @@ fn reset_view_state(window: &AppWindow) {
     // previous image's history.
     window.set_can_undo(false);
     window.set_can_redo(false);
+    // TRACES: FR-DSP-7
+    // Emptied rather than left standing: the previous photograph's histogram
+    // beside the next one's filename is a confident, precise lie, and the gap
+    // before the new frame settles is exactly long enough to read it.
+    window.set_histogram(histogram::empty());
 }
 
 /// Push the framing back to the geometry panel.
@@ -1016,10 +1022,32 @@ pub fn run(paths: Vec<PathBuf>) -> Result<()> {
                     // duration of every gesture.
                     let (vw, vh) = *viewport.borrow();
                     window.set_magnified(s.magnifies_source(vw, vh));
+
+                    // TRACES: FR-DSP-7
+                    // **Counted on the settled frame and no other.**
+                    //
+                    // FR-DSP-7 requires the histogram not extend the FR-DSP-3
+                    // frame budget, and `draft` is already exactly the flag
+                    // that says a gesture is still moving — so this reduction
+                    // and its 4 KB transfer happen once when the slider stops
+                    // rather than on every one of the forty frames a drag
+                    // emits. Nothing is lost by it: a histogram of a
+                    // half-resolution frame flickering past under a finger is
+                    // not a reading anyone takes.
+                    if !draft {
+                        window.set_histogram(
+                            s.histogram()
+                                .as_ref()
+                                .map_or_else(histogram::empty, histogram::view),
+                        );
+                    }
                 }
                 Err(e) => {
                     log::warn!("render failed: {e}");
                     window.set_load_error(e.into());
+                    // No frame, so nothing to describe. The stale plot would
+                    // otherwise sit beside the error message looking current.
+                    window.set_histogram(histogram::empty());
                 }
             }
         })