fix: prevent match group expanders from collapsing on button click

Replace st.rerun() with on_click callbacks so decisions write to
session state before the natural rerun. Decided groups auto-collapse
with status in the label; undecided groups stay expanded. Added undo
button on decided groups.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/gui/app.py

@@ -213,11 +213,7 @@ if uploaded is not None:
                 # Individual group cards
                 decisions = st.session_state["review_decisions"]
                 for i, group in enumerate(result.match_groups):
-                    decision = match_group_card(group, df, group_num=i + 1)
-                    if decision is not None:
-                        decisions[group.group_id] = decision
-                        st.session_state["review_decisions"] = decisions
-                        st.rerun()
+                    match_group_card(group, df, group_num=i + 1)
 
                 # Show decision summary
                 if decisions:

src/gui/components.py

@@ -262,26 +262,37 @@ def match_group_card(
     group: MatchResult,
     df: pd.DataFrame,
     group_num: int,
-) -> Optional[bool]:
+) -> None:
     """Render an expandable match group card with side-by-side diff.
 
-    Returns:
-        True  — user clicked Merge (accept match)
-        False — user clicked Keep Both (reject match)
-        None  — no decision yet
+    Decisions are stored directly in ``st.session_state["review_decisions"]``
+    via ``on_click`` callbacks so that other expanders keep their state on
+    rerun.
     """
     confidence = group.confidence
-    auto_expand = confidence < 95.0
     matched_on = ", ".join(group.matched_on)
     n_rows = len(group.row_indices)
+    gid = group.group_id
 
+    decisions = st.session_state.get("review_decisions", {})
+    has_decision = gid in decisions
+    decision_val = decisions.get(gid)
+
+    # Build label — append decision status if already decided
     label = (
         f"Group {group_num}: {n_rows} rows "
         f"(confidence: {confidence:.0f}%) "
         f"[{matched_on}]"
     )
+    if decision_val is True:
+        label += " — Merged"
+    elif decision_val is False:
+        label += " — Kept Both"
 
-    with st.expander(label, expanded=auto_expand):
+    # Decided groups collapse; undecided groups stay open
+    expanded = not has_decision
+
+    with st.expander(label, expanded=expanded):
         # Build comparison DataFrame
         display_cols = [c for c in df.columns if not str(c).startswith("_norm_")]
         rows_data = []
@@ -312,28 +323,32 @@ def match_group_card(
         styled = compare_df.style.apply(_highlight_diffs, axis=0)
         st.dataframe(styled, use_container_width=True)
 
-        # Action buttons
-        btn_left, btn_mid, btn_right = st.columns(3)
-        merge_key = f"merge_{group.group_id}"
-        keep_key = f"keep_{group.group_id}"
-
-        with btn_left:
-            if st.button("Merge", key=merge_key, type="primary"):
-                return True
-        with btn_mid:
-            if st.button("Keep Both", key=keep_key):
-                return False
-
-        # Check session state for previous decisions
-        decisions = st.session_state.get("review_decisions", {})
-        if group.group_id in decisions:
-            decision = decisions[group.group_id]
-            if decision is True:
+        if has_decision:
+            # Show current decision with option to undo
+            if decision_val is True:
                 st.success("Decision: Merge")
-            elif decision is False:
+            else:
                 st.info("Decision: Keep Both")
 
-    return None
+            def _undo(g=gid):
+                st.session_state["review_decisions"].pop(g, None)
+
+            st.button("Undo", key=f"undo_{gid}", on_click=_undo)
+        else:
+            # Action buttons — on_click writes to session state before rerun
+            def _on_merge(g=gid):
+                st.session_state["review_decisions"][g] = True
+
+            def _on_keep(g=gid):
+                st.session_state["review_decisions"][g] = False
+
+            btn_left, btn_mid, _btn_right = st.columns(3)
+            with btn_left:
+                st.button("Merge", key=f"merge_{gid}",
+                          type="primary", on_click=_on_merge)
+            with btn_mid:
+                st.button("Keep Both", key=f"keep_{gid}",
+                          on_click=_on_keep)
 
 
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