diff --git a/src/cloudbox.rs b/src/cloudbox.rs
index d490457..baa5591 100644
--- a/src/cloudbox.rs
+++ b/src/cloudbox.rs
@@ -2092,6 +2092,17 @@ fn box_look(script: Option<&str>, after: Option<String>, pointer: bool) -> Resul
     } else {
         text.push_str(&format!("\n可以操作的東西（圖上同樣的編號，用 ref 點）：\n{}", lines.join("\n")));
     }
+    // What moved since the last look (same signal the Mac path gives the quick model).
+    static LAST_ELS: Mutex<Vec<crate::marks::Element>> = Mutex::new(Vec::new());
+    {
+        let cur: Vec<crate::marks::Element> = numbered.iter().map(|(_, e)| e.clone()).collect();
+        let mut last = LAST_ELS.lock().unwrap_or_else(|e| e.into_inner());
+        let d = crate::marks::diff(&last, &cur);
+        *last = cur;
+        if !d.is_empty() {
+            text.push_str(&format!("\n跟上一張圖相比：\n{}", d.join("\n")));
+        }
+    }
     // Step timing (for the 電腦 card): how long the box's side took vs. the whole step.
     {
         let total_ms = t_all.elapsed().as_millis() as u64;
diff --git a/src/computer.rs b/src/computer.rs
index c511067..07a8f6f 100644
--- a/src/computer.rs
+++ b/src/computer.rs
@@ -1169,6 +1169,17 @@ fn look(after: Option<String>, pointer: bool) -> Result<(String, Option<String>)
         text.push_str(&format!("\n讀不到最前面「{front}」視窗裡的按鈕{why}：下面只有選單列。視窗裡的東西照圖上的位置點 x、y，或等一下再截一次圖。\n{}", lines.join("\n")));
     } else {
         text.push_str(&format!("\n「{app}」上可以操作的東西（圖上同樣的編號，用 ref 點）：\n{}", lines.join("\n")));
+        // What moved since the last look (the quick model's step-effect signal).
+        static LAST_ELS: Mutex<Vec<crate::marks::Element>> = Mutex::new(Vec::new());
+        {
+            let cur: Vec<crate::marks::Element> = numbered.iter().map(|(_, e)| e.clone()).collect();
+            let mut last = LAST_ELS.lock().unwrap_or_else(|e| e.into_inner());
+            let d = crate::marks::diff(&last, &cur);
+            *last = cur;
+            if !d.is_empty() {
+                text.push_str(&format!("\n跟上一張圖相比：\n{}", d.join("\n")));
+            }
+        }
     }
     text.push_str(a11y_note);
     Ok((text, img_b64))
diff --git a/src/marks.rs b/src/marks.rs
index 4b0e0ce..532104c 100644
--- a/src/marks.rs
+++ b/src/marks.rs
@@ -243,6 +243,40 @@ fn describe(e: &Element) -> String {
 }
 
 /// A fingerprint of what the controls are and where (to say "nothing changed").
+/// What came and what went between two looks at the screen (multiset of (kind, name)).
+/// The quick model reads this to know what its last step did, without re-reading the whole
+/// numbered list (the list is long; the change is usually two lines).
+pub fn diff(prev: &[Element], now: &[Element]) -> Vec<String> {
+    fn key(e: &Element) -> (&str, &str) {
+        (e.kind.as_str(), e.name.as_str())
+    }
+    let mut counts: HashMap<(&str, &str), i32> = HashMap::new();
+    for e in prev {
+        *counts.entry(key(e)).or_default() += 1;
+    }
+    for e in now {
+        *counts.entry(key(e)).or_default() -= 1;
+    }
+    let mut lines = Vec::new();
+    // Order: as they appear in the current picture (adds), then as they were listed (removes).
+    let mut shown: std::collections::HashSet<(&str, &str)> = std::collections::HashSet::new();
+    for e in now {
+        let k = key(e);
+        if counts.get(&k) == Some(&-1) && shown.insert(k) {
+            lines.push(format!("＋ {}「{}」", e.kind, e.name));
+        }
+    }
+    for e in prev {
+        let k = key(e);
+        if counts.get(&k) == Some(&1) && shown.insert(k) {
+            lines.push(format!("－ {}「{}」", e.kind, e.name));
+        }
+    }
+    // A dialog that appeared entirely is the case that matters: show it in full.
+    lines.truncate(12);
+    lines
+}
+
 pub fn fingerprint(elements: &[Element]) -> u64 {
     use std::hash::{Hash, Hasher};
     let mut h = std::collections::hash_map::DefaultHasher::new();
@@ -348,6 +382,22 @@ fn digit(img: &mut image::RgbImage, x: i64, y: i64, d: u8, c: [u8; 3]) {
 mod tests {
     use super::*;
 
+    fn mel(kind: &str, name: &str) -> Element {
+        Element { kind: kind.into(), name: name.into(), value: String::new(), x: 0.0, y: 0.0, w: 10.0, h: 10.0, active: true, flags: vec![], window: String::new() }
+    }
+
+    #[test]
+    fn the_change_between_two_looks_is_two_lines() {
+        let before = vec![mel("按鈕", "Save"), mel("輸入框", "標題")];
+        let after = vec![mel("按鈕", "Save"), mel("按鈕", "OK"), mel("按鈕", "Cancel")];
+        assert_eq!(diff(&before, &after), ["＋ 按鈕「OK」", "＋ 按鈕「Cancel」", "－ 輸入框「標題」"]);
+        // Same controls twice = a multiset: one of two identical buttons leaving is a change.
+        let two = vec![mel("按鈕", "x"), mel("按鈕", "x")];
+        assert_eq!(diff(&two, &[mel("按鈕", "x")]), ["－ 按鈕「x」"]);
+        assert!(diff(&before, &before).is_empty());
+    }
+
+
     fn el(kind: &str, name: &str, x: f64, y: f64, active: bool) -> Element {
         Element { kind: kind.into(), name: name.into(), value: String::new(), x, y, w: 40.0, h: 20.0, active, flags: vec![], window: "W".into() }
     }