diff --git a/app-proto/src/assembly.rs b/app-proto/src/assembly.rs
index b4b5329..278a8f9 100644
--- a/app-proto/src/assembly.rs
+++ b/app-proto/src/assembly.rs
@@ -317,36 +317,50 @@ impl Assembly {
             console::log_1(&JsValue::from("The assembly is rigid"));
         }
         
-        const ELEMENT_DIM: usize = 5;
-        let assembly_dim = self.elements.with(|elts| elts.len());
-        let mut motion_proj = DMatrix::zeros(ELEMENT_DIM, assembly_dim);
-        let tan_assembly_dim = self.tangent.with(|tan| tan.assembly_dim());
-        
-        // project the element motions onto the tangent space of the solution
-        // variety, and sum them to get a deformation of the whole assembly
-        let mut next_column_index = tan_assembly_dim;
-        for elt_motion in motion {
-            match self.elements.with_untracked(|elts| elts[elt_motion.key].column_index) {
-                Some(column_index) => {
-                    let mut target_columns = motion_proj.columns_mut(0, tan_assembly_dim);
-                    target_columns += self.tangent.with(
-                        |tan| tan.proj(&elt_motion.velocity, column_index)
-                    )
-                },
-                None => {
-                    // give the element a column index, even though it's never
-                    // been through a realization, and add its motion to that
-                    // column of the projected motion. this temporarily breaks
-                    // invariant (1), but the invariant will be restored when we
-                    // realize the assembly at the end of the deformation
-                    let mut target_column = motion_proj.column_mut(next_column_index);
-                    target_column += elt_motion.velocity;
-                    self.elements.update_silent(
-                        |elts| elts[elt_motion.key].column_index = Some(next_column_index)
-                    );
+        // give a column index to each moving element that doesn't have one yet.
+        // this temporarily breaks invariant (1), but the invariant will be
+        // restored when we realize the assembly at the end of the deformation.
+        // in the process, we find out how many matrix columns we'll need to
+        // hold the deformation
+        let realized_dim = self.tangent.with(|tan| tan.assembly_dim());
+        let motion_dim = self.elements.update_silent(|elts| {
+            let mut next_column_index = realized_dim;
+            for elt_motion in motion.iter() {
+                let moving_elt = &mut elts[elt_motion.key];
+                if moving_elt.column_index.is_none() {
+                    moving_elt.column_index = Some(next_column_index);
                     next_column_index += 1;
                 }
             }
+            next_column_index
+        });
+        
+        // project the element motions onto the tangent space of the solution
+        // variety and sum them to get a deformation of the whole assembly. the
+        // matrix `motion_proj` that holds the deformation has extra columns for
+        // any moving elements that aren't reflected in the saved tangent space
+        const ELEMENT_DIM: usize = 5;
+        let mut motion_proj = DMatrix::zeros(ELEMENT_DIM, motion_dim);
+        for elt_motion in motion {
+            // we can unwrap the column index because we know that every moving
+            // element has one at this point
+            let column_index = self.elements.with_untracked(
+                |elts| elts[elt_motion.key].column_index.unwrap()
+            );
+            
+            if column_index < realized_dim {
+                // this element had a column index when we started, so by
+                // invariant (1), it's reflected in the tangent space
+                let mut target_columns = motion_proj.columns_mut(0, realized_dim);
+                target_columns += self.tangent.with(
+                    |tan| tan.proj(&elt_motion.velocity, column_index)
+                );
+            } else {
+                // this element didn't have a column index when we started, so
+                // by invariant (2), it's unconstrained
+                let mut target_column = motion_proj.column_mut(column_index);
+                target_column += elt_motion.velocity;
+            }
         }
         
         // step each element along the mass shell geodesic that matches its