Sketcher: Use DenseQR for small sketches to avoid SparseQR rank issues (#26559)

src/Mod/Sketcher/App/Sketch.h

@@ -641,6 +641,14 @@ public:
     {
         return debugMode;
     }
+    inline void setAutoQRThreshold(int val)
+    {
+        GCSsys.autoQRThreshold = val;
+    }
+    inline void setSketchAutoAlgo(bool val)
+    {
+        GCSsys.autoChooseAlgorithm = val;
+    }
     inline void setMaxIter(int maxiter)
     {
         GCSsys.maxIter = maxiter;

src/Mod/Sketcher/App/planegcs/GCS.cpp

@@ -486,6 +486,8 @@ System::System()
     , convergence(1e-10)
     , convergenceRedundant(1e-10)
     , qrAlgorithm(EigenSparseQR)
+    , autoChooseAlgorithm(true)
+    , autoQRThreshold(1000)
     , dogLegGaussStep(FullPivLU)
     , qrpivotThreshold(1E-13)
     , debugMode(Minimal)
@@ -4808,6 +4810,15 @@ int System::diagnose(Algorithm alg)
     hasDiagnosis = true;
     dofs = pdiagnoselist.size();
 
+    // Use DenseQR for small to medium systems to avoid SparseQR rank issues.
+    // SparseQR is known to fail rank detection on specific geometric structures (e.g. aligned slots).
+    // 200 parameters roughly corresponds to ~100 points/curves, covering most complex sketches
+    // where stability is preferred over pure O(N) performance.
+    // See: https://github.com/FreeCAD/FreeCAD/issues/10903
+    if (autoChooseAlgorithm) {
+        qrAlgorithm = dofs < autoQRThreshold ? EigenDenseQR : EigenSparseQR;
+    }
+
     // There is a legacy decision to use QR decomposition. I (abdullah) do not know all the
     // consideration taken in that decisions. I see that:
     // - QR decomposition is able to provide information about the rank and

src/Mod/Sketcher/App/planegcs/GCS.h

@@ -239,6 +239,8 @@ public:
     double convergence;
     double convergenceRedundant;
     QRAlgorithm qrAlgorithm;
+    bool autoChooseAlgorithm;
+    int autoQRThreshold;
     DogLegGaussStep dogLegGaussStep;
     double qrpivotThreshold;
     DebugMode debugMode;