fix: preserve staff-scoped staff-layout number="N"

src/include/mx/api/SystemLayoutData.h

@@ -9,6 +9,7 @@
 #include "mx/api/LeftRight.h"
 #include "mx/api/PageLayoutData.h"
 
+#include <map>
 #include <string>
 #include <vector>
 
@@ -29,21 +30,29 @@ class SystemLayoutData
     /// Distance from the top margin of the page to the top line of the first staff on the page, in tenths.
     OptionalDouble topSystemDistance;
 
-    /// the space between staves within the same system, in tenths.
+    /// The space between staves within the same system, in tenths. Applies to every staff of a part
+    /// that does not have its own entry in staffDistances.
     OptionalDouble staffDistance;
 
+    /// Per-staff spacing for multi-staff parts, keyed by zero-based staff index: the space between
+    /// the bottom line of the previous staff and the top line of the keyed staff, in tenths.
+    /// Normally empty; only add an entry when one staff needs its own spacing, e.g. extra room
+    /// above an organ's pedal staff. For a given staff, an entry here wins; otherwise
+    /// staffDistance applies.
+    std::map<int, double> staffDistances;
+
     /// Returns true if any of the members have values.
     inline bool isUsed() const
     {
-        return margins || systemDistance || topSystemDistance || staffDistance;
+        return margins || systemDistance || topSystemDistance || staffDistance || !staffDistances.empty();
     }
 
     explicit inline SystemLayoutData(std::optional<LeftRight> inMargins = std::nullopt,
                                      OptionalDouble inSystemDistance = std::nullopt,
                                      OptionalDouble inTopSystemDistance = std::nullopt,
                                      OptionalDouble inStaffDistance = std::nullopt)
         : margins(inMargins), systemDistance(inSystemDistance), topSystemDistance{inTopSystemDistance},
-          staffDistance(inStaffDistance)
+          staffDistance(inStaffDistance), staffDistances{}
     {
     }
 };
@@ -53,6 +62,7 @@ MXAPI_EQUALS_MEMBER(margins)
 MXAPI_EQUALS_MEMBER(systemDistance)
 MXAPI_EQUALS_MEMBER(topSystemDistance)
 MXAPI_EQUALS_MEMBER(staffDistance)
+MXAPI_EQUALS_MEMBER(staffDistances)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(SystemLayoutData);
 } // namespace api

src/private/mx/impl/LayoutFunctions.cpp

@@ -236,6 +236,15 @@ void addSystemMargins(const api::DefaultsData &inDefaults, core::ScoreHeaderGrou
         needsDefaults = true;
     }
 
+    for (const auto &[staffIndex, staffDistance] : inDefaults.systemLayout.staffDistances)
+    {
+        core::StaffLayout staffLayout;
+        staffLayout.setNumber(core::StaffNumber{staffIndex + 1});
+        staffLayout.setStaffDistance(toTenths(staffDistance));
+        layout.addStaffLayout(staffLayout);
+        needsDefaults = true;
+    }
+
     if (needsDefaults)
     {
         layout.setSystemLayout(systemLayout);
@@ -427,15 +436,24 @@ void addStaffLayout(const core::ScoreHeaderGroup &inScoreHeaderGroup, api::Defau
         return;
     }
 
-    const auto &staffLayouts = inScoreHeaderGroup.defaults()->layout().staffLayout();
-    if (staffLayouts.empty())
-    {
-        return;
-    }
-
-    if (staffLayouts.front().staffDistance().has_value())
+    // A source with an explicit number attribute is staff-scoped (a staffDistances map entry);
+    // one with no number applies to staff 1 by schema default and is captured as the plain
+    // unscoped value. Mirrors ScoreReader::scanForSystemInfo's per-measure handling.
+    for (const auto &staffLayout : inScoreHeaderGroup.defaults()->layout().staffLayout())
     {
-        outDefaults.systemLayout.staffDistance = staffLayouts.front().staffDistance()->value().value();
+        if (!staffLayout.staffDistance().has_value())
+        {
+            continue;
+        }
+        const double distance = staffLayout.staffDistance()->value().value();
+        if (staffLayout.number().has_value())
+        {
+            outDefaults.systemLayout.staffDistances[staffLayout.number()->value() - 1] = distance;
+        }
+        else
+        {
+            outDefaults.systemLayout.staffDistance = distance;
+        }
     }
 }
 

src/private/mx/impl/MeasureWriter.cpp

@@ -26,6 +26,7 @@
 #include "mx/core/generated/Repeat.h"
 #include "mx/core/generated/RightLeftMiddle.h"
 #include "mx/core/generated/StaffLayout.h"
+#include "mx/core/generated/StaffNumber.h"
 #include "mx/core/generated/SystemLayout.h"
 #include "mx/core/generated/SystemMargins.h"
 #include "mx/core/generated/Tenths.h"
@@ -263,6 +264,15 @@ void MeasureWriter::writeSystemInfo()
             outLayoutGroup.addStaffLayout(outStaffLayout);
             outPrint.setLayout(outLayoutGroup);
         }
+
+        for (const auto &[staffIndex, staffDistance] : inSystemLayout.staffDistances)
+        {
+            core::StaffLayout outStaffLayout{};
+            outStaffLayout.setNumber(core::StaffNumber{staffIndex + 1});
+            outStaffLayout.setStaffDistance(core::Tenths{core::Decimal{staffDistance}});
+            outLayoutGroup.addStaffLayout(outStaffLayout);
+            outPrint.setLayout(outLayoutGroup);
+        }
     }
 
     myOutMeasure.addMusicData(core::MusicDataChoice::print(outPrint));

src/private/mx/impl/ScoreReader.cpp

@@ -466,15 +466,23 @@ void ScoreReader::scanForSystemInfo() const
                 }
             }
 
-            // Per-measure <staff-layout> staff-distance. (We model a single
-            // staff-distance, matching how the defaults staff-layout is
-            // mapped; the first staff-layout's distance is used.)
+            // Per-measure <staff-layout> staff-distance. A source with an explicit number
+            // attribute is staff-scoped (a staffDistances map entry); one with no number applies
+            // to staff 1 by schema default and is captured as the plain unscoped value.
             for (const auto &staffLayout : layoutGroup.staffLayout())
             {
-                if (staffLayout.staffDistance().has_value())
+                if (!staffLayout.staffDistance().has_value())
                 {
-                    systemData.layout.staffDistance = staffLayout.staffDistance()->value().value();
-                    break;
+                    continue;
+                }
+                const double distance = staffLayout.staffDistance()->value().value();
+                if (staffLayout.number().has_value())
+                {
+                    systemData.layout.staffDistances[staffLayout.number()->value() - 1] = distance;
+                }
+                else
+                {
+                    systemData.layout.staffDistance = distance;
                 }
             }
 

src/private/mxtest/api/PrintLayoutRoundTripTest.cpp

@@ -71,4 +71,32 @@ TEST(printLayoutRoundTrip, perMeasureStaffDistance)
     CHECK_EQUAL(120.0, sysOut.layout.systemDistance.value());
 }
 
+// A <staff-layout number="N"> is scoped to one staff of a multi-staff part; the writer
+// only ever emitted an unscoped <staff-layout> (matching SystemLayoutData::staffDistance),
+// and the reader dropped the number attribute on the one it read back, silently
+// misattributing the distance to staff 1. Verify a staff-scoped distance survives via
+// SystemLayoutData::staffDistances.
+TEST(printLayoutRoundTrip, staffScopedStaffDistance)
+{
+    auto in = makeScoreWithMeasures(1);
+    auto &staff0 = in.parts.front().measures.front().staves.front();
+    StaffData staff1 = staff0;
+    in.parts.front().measures.front().staves.push_back(staff1);
+
+    SystemData sys{};
+    sys.layout.staffDistances[1] = 65.0;
+    in.layout.emplace(0, LayoutData{sys, PageData{}});
+
+    const auto out = mxtest::roundTrip(in);
+
+    const auto it = out.layout.find(0);
+    REQUIRE(it != out.layout.end());
+    const auto &sysOut = it->second.system;
+    CHECK(!static_cast<bool>(sysOut.layout.staffDistance));
+    CHECK_EQUAL(1, static_cast<int>(sysOut.layout.staffDistances.size()));
+    const auto sdIt = sysOut.layout.staffDistances.find(1);
+    REQUIRE(sdIt != sysOut.layout.staffDistances.end());
+    CHECK_EQUAL(65.0, sdIt->second);
+}
+
 #endif