fix(eeschema): collab drag no longer drops segments — batch one edit into one delta

A single SCH_COMMIT::Push fires OnItemsAdded/Removed/Changed separately and
synchronously, then RecalculateConnections once. COLLAB_LISTENER emitted each
category as its own delta, so the peer applied one atomic edit as three separate
commits, each with its own connectivity recompute. On a large connected drag the
junction at the wires' new crossing (added) was applied before the wires moved
(changed) -> dangling junction -> the peer's cleanup deleted it (lost segments).

COLLAB_LISTENER now buffers added/changed/removed (serialized in each synchronous
callback) and flushes one combined delta after Push returns, coalesced via
CallAfter. doApply applies it atomically removed->changed->added in a single
commit with one recompute, so the junction survives. Verified two-tab: a G-drag
of U1A that previously left the peer at 74 items / 7 junctions now emits one
delta {a:1,c:4,r:1} and both tabs converge at 75 / 8.

Root-repo only; kicad and wxwidgets untouched.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

features/yjs-bridge/0007-eeschema-essential-ops-findings.md

@@ -87,6 +87,30 @@ produce the s-expr clipboard blob (`SCH_IO_KICAD_SEXPR::Format` →
 until symbol libraries are bundled (you can't place a symbol natively to sync). Symbol
 **move/position** already syncs via the generic `changed` path.
 
+## Thrust C — collab wire/symbol-drag divergence (lost segments) — fixed
+
+User-reported follow-up: dragging a wire OR symbol with multiple connections a *large*
+distance (small drags don't reroute) lost segments on the peer. Reproduced + root-caused
+in the real app, two tabs.
+
+**Root cause:** one atomic edit (a single `SCH_COMMIT::Push`) calls `OnItemsAdded`,
+`OnItemsRemoved`, `OnItemsChanged` **separately and synchronously**, then
+`RecalculateConnections` once. The old `COLLAB_LISTENER` emitted each category as its own
+delta, so the peer applied them as THREE separate commits, each with its own connectivity
+recompute. A `G`-drag of U1A emitted `{added:[junction]}`, `{removed:[wire]}`,
+`{changed:[3 wires, symbol]}`; the junction (at the wires' new crossing) was applied
+*before* the wires moved → a **dangling junction on the peer → connectivity cleanup deleted
+it**. Result: tab A 75 items / 8 junctions, tab B 74 / 7 (the junction lost). Simple
+translates (`M` tool) always converged — they touch only `changed`.
+
+**Fix (`wasm/bindings/eeschema_embind.cpp`):** `COLLAB_LISTENER` now buffers the three
+categories (serializing items in each synchronous callback) and flushes **one combined
+delta** after Push returns, coalesced via `CallAfter`. The peer's `doApply` applies a
+combined delta removed→changed→added in a single `SCH_COMMIT` with one recompute, so the
+junction is added after its wires are in place and survives. **Verified:** the same G-drag
+now emits 1 delta `{a:1,c:4,r:1}` and both tabs converge identically (75 items, 8 junctions,
+zero wire/junction diff). Embind-only build; eeschema-collab + eeschema-ui suites green.
+
 ## Files touched (all root repo)
 
 - `web/apps/frontend/src/index.css` — `.window` / `.window-canvas` CSS (Thrust A).

wasm/bindings/eeschema_embind.cpp

@@ -286,17 +286,30 @@ void emit( const json& aDelta )
 
 // ChangeSource: native SCHEMATIC_LISTENER. SCH_COMMIT::Push fires these in bulk for
 // every local edit (move, add, remove, …) — that's our emit trigger.
+//
+// CRUCIAL: one local edit (a single SCH_COMMIT::Push) calls OnItemsAdded, OnItemsRemoved
+// and OnItemsChanged *separately and synchronously*, then runs RecalculateConnections once
+// (sch_commit.cpp). Emitting each category as its own delta makes the peer apply them as
+// THREE separate commits, each followed by its own connectivity recompute — so an item that
+// only makes sense in the final state is mis-handled mid-sequence. Concretely, a connected
+// drag adds a junction at the wires' new crossing AND moves those wires; if the junction
+// (added) is applied before the wires (changed), the peer sees a dangling junction and
+// connectivity cleanup deletes it → the peer permanently loses it (the "lost segments on a
+// big drag" divergence). Fix: buffer all three categories and emit ONE combined delta after
+// Push returns (coalesced via CallAfter), so the peer applies it atomically in a single
+// SCH_COMMIT — doApply orders it removed→changed→added, with one recompute at the end, so the
+// junction is added after its wires are in place and survives.
 class COLLAB_LISTENER : public SCHEMATIC_LISTENER
 {
 public:
     void OnSchItemsAdded( SCHEMATIC&, std::vector<SCH_ITEM*>& aItems ) override
     {
-        emitItems( "added", aItems );
+        accumulate( m_added, aItems );
     }
 
     void OnSchItemsChanged( SCHEMATIC&, std::vector<SCH_ITEM*>& aItems ) override
     {
-        emitItems( "changed", aItems );
+        accumulate( m_changed, aItems );
     }
 
     void OnSchItemsRemoved( SCHEMATIC&, std::vector<SCH_ITEM*>& aItems ) override
@@ -304,35 +317,60 @@ public:
         if( s_applyingRemote )
             return;
 
-        json removed = json::array();
-
         for( SCH_ITEM* item : aItems )
-            removed.push_back( toUtf8( item->m_Uuid.AsString() ) );
+            m_removed.push_back( toUtf8( item->m_Uuid.AsString() ) );
 
-        if( !removed.empty() )
-            emit( json{ { "added", json::array() }, { "changed", json::array() },
-                        { "removed", removed } } );
+        scheduleFlush();
     }
 
 private:
-    void emitItems( const char* aKey, std::vector<SCH_ITEM*>& aItems )
+    // Serialize the items NOW (they're valid during the synchronous callback); the buffered
+    // json is emitted later by flush().
+    void accumulate( json& aBucket, std::vector<SCH_ITEM*>& aItems )
     {
         if( s_applyingRemote )
             return;
 
-        json arr = json::array();
-
         for( SCH_ITEM* item : aItems )
-            arr.push_back( itemToJson( item ) );
+            aBucket.push_back( itemToJson( item ) );
 
-        if( arr.empty() )
+        scheduleFlush();
+    }
+
+    // All three category callbacks for one Push fire synchronously before control returns to
+    // the main loop, so a single CallAfter run after Push drains them into one delta. (Several
+    // commits in the same loop turn coalesce into one delta — harmless for the CRDT.)
+    void scheduleFlush()
+    {
+        if( m_flushScheduled )
             return;
 
-        json d = { { "added", json::array() }, { "changed", json::array() },
-                   { "removed", json::array() } };
-        d[aKey] = arr;
-        emit( d );
+        m_flushScheduled = true;
+
+        if( SCH_EDIT_FRAME* fr = schFrame() )
+            fr->CallAfter( [this]() { flush(); } );
+        else
+            flush();
     }
+
+    void flush()
+    {
+        m_flushScheduled = false;
+
+        if( m_added.empty() && m_changed.empty() && m_removed.empty() )
+            return;
+
+        emit( json{ { "added", m_added }, { "changed", m_changed }, { "removed", m_removed } } );
+
+        m_added = json::array();
+        m_changed = json::array();
+        m_removed = json::array();
+    }
+
+    json m_added = json::array();
+    json m_changed = json::array();
+    json m_removed = json::array();
+    bool m_flushScheduled = false;
 };
 
 COLLAB_LISTENER* g_listener = nullptr;