From a0e5e6eb14d417854b155bc2aa571408f43c44d9 Mon Sep 17 00:00:00 2001
From: David Mann <daaavid.mann@gmail.com>
Date: Wed, 14 Oct 2026 19:49:52 +0200
Subject: [PATCH] Count tool: deleting an earlier count group keeps the active
 group (#5653)

count.deleteGroup only clamped the active index to the new length, so
removing a group before the active one silently made the next group
active and later count.add calls landed there. Shift the index down
when the removed group precedes it; deleting the active last group
still selects the new last group.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
---
 crates/engine/src/analysis_cmds.rs       |  4 +++-
 crates/engine/src/analysis_cmds/tests.rs | 21 +++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/crates/engine/src/analysis_cmds.rs b/crates/engine/src/analysis_cmds.rs
index 6ad25951db..7cfd07e48e 100644
--- a/crates/engine/src/analysis_cmds.rs
+++ b/crates/engine/src/analysis_cmds.rs
@@ -856,7 +856,9 @@ fn count_delete_group(s: &mut Session, p: &Value) -> Result<Value> {
     s.edit("Delete Count Group", |d, _| {
         d.measurement.count_groups.remove(g);
         let n = d.measurement.count_groups.len();
-        d.measurement.active_count_group = d.measurement.active_count_group.min(n.saturating_sub(1));
+        // Removing a group before the active one shifts it down a slot.
+        let a = d.measurement.active_count_group;
+        d.measurement.active_count_group = (if g < a { a - 1 } else { a }).min(n.saturating_sub(1));
         Ok(())
     })?;
     Ok(count_json(doc(s)?.as_ref()))
diff --git a/crates/engine/src/analysis_cmds/tests.rs b/crates/engine/src/analysis_cmds/tests.rs
index 124f9abf1e..6c1c6b6242 100644
--- a/crates/engine/src/analysis_cmds/tests.rs
+++ b/crates/engine/src/analysis_cmds/tests.rs
@@ -291,6 +291,27 @@ fn count_groups_markers_and_undo() {
     assert!(s.execute("count.add", json!({"x": 1})).is_err());
 }
 
+#[test]
+fn deleting_an_earlier_count_group_keeps_the_active_group() {
+    let mut s = session(100, 100, 8);
+    s.execute("count.add", json!({"x": 10, "y": 10})).unwrap();
+    for _ in 0..3 {
+        s.execute("count.newGroup", json!({})).unwrap();
+    }
+    s.execute("count.setGroup", json!({"group": 2})).unwrap();
+    let r = s.execute("count.deleteGroup", json!({"group": 0})).unwrap();
+    assert_eq!(r["activeGroup"], 1);
+    assert_eq!(r["groups"][1]["name"], "Count Group 3");
+    // A later group leaves the active index alone.
+    let r = s.execute("count.deleteGroup", json!({"group": 2})).unwrap();
+    assert_eq!(r["activeGroup"], 1);
+    assert_eq!(r["groups"][1]["name"], "Count Group 3");
+    // Deleting the active last group selects the new last group.
+    let r = s.execute("count.deleteGroup", json!({})).unwrap();
+    assert_eq!(r["activeGroup"], 0);
+    assert_eq!(r["groups"][0]["name"], "Count Group 2");
+}
+
 #[test]
 fn log_delete_and_csv_export() {
     let mut s = session(10, 10, 8);