Fix text overwriting end of box for empty default case in mpl drawer (backport #11366)

qiskit/visualization/circuit/matplotlib.py

@@ -778,14 +778,21 @@ def _get_coords(
                 l_width.append(layer_widths[node][0])
                 node_data[node].x_index = x_index
 
+                # Special case of default case with no ops in it, need to push end
+                # of switch op one extra x_index
+                if isinstance(node.op, SwitchCaseOp):
+                    if len(node.op.blocks[-1]) == 0:
+                        curr_x_index += 1
+
             # adjust the column if there have been barriers encountered, but not plotted
             barrier_offset = 0
             if not self._plot_barriers:
                 # only adjust if everything in the layer wasn't plotted
                 barrier_offset = (
                     -1 if all(getattr(nd.op, "_directive", False) for nd in layer) else 0
                 )
-            prev_x_index = curr_x_index + max(l_width) + barrier_offset - 1
+            max_lwidth = max(l_width) if l_width else 0
+            prev_x_index = curr_x_index + max_lwidth + barrier_offset - 1
 
         return prev_x_index + 1
 
@@ -1505,7 +1512,6 @@ def _flow_op_gate(self, node, node_data, glob_data):
 
         if_width = node_data[node].width[0] + WID
         box_width = if_width
-
         # Add the else and case widths to the if_width
         for ewidth in node_data[node].width[1:]:
             if ewidth > 0.0:
@@ -1532,19 +1538,6 @@ def _flow_op_gate(self, node, node_data, glob_data):
             y_shift = fold_level * (glob_data["n_lines"] + 1)
             end_x = xpos + box_width - x_shift
 
-            # FancyBbox allows rounded corners
-            box = glob_data["patches_mod"].FancyBboxPatch(
-                xy=(xpos - x_shift, ypos - 0.5 * HIG - y_shift),
-                width=box_width,
-                height=height,
-                boxstyle="round, pad=0.1",
-                fc="none",
-                ec=colors[node_data[node].nest_depth % 4],
-                linewidth=self._lwidth3,
-                zorder=PORDER_FLOW,
-            )
-            self._ax.add_patch(box)
-
             if isinstance(node.op, IfElseOp):
                 flow_text = "  If"
             elif isinstance(node.op, WhileLoopOp):
@@ -1554,12 +1547,32 @@ def _flow_op_gate(self, node, node_data, glob_data):
             elif isinstance(node.op, SwitchCaseOp):
                 flow_text = "Switch"
 
+            # Some spacers. op_spacer moves 'Switch' back a bit for alignment,
+            # expr_spacer moves the expr over to line up with 'Switch' and
+            # empty_default_spacer makes the switch box longer if the default
+            # case is empty so text doesn't run past end of box.
             if isinstance(node.op, SwitchCaseOp):
                 op_spacer = 0.04
                 expr_spacer = 0.0
+                empty_default_spacer = 0.3 if len(node.op.blocks[-1]) == 0 else 0.0
             else:
                 op_spacer = 0.08
                 expr_spacer = 0.02
+                empty_default_spacer = 0.0
+
+            # FancyBbox allows rounded corners
+            box = glob_data["patches_mod"].FancyBboxPatch(
+                xy=(xpos - x_shift, ypos - 0.5 * HIG - y_shift),
+                width=box_width + empty_default_spacer,
+                height=height,
+                boxstyle="round, pad=0.1",
+                fc="none",
+                ec=colors[node_data[node].nest_depth % 4],
+                linewidth=self._lwidth3,
+                zorder=PORDER_FLOW,
+            )
+            self._ax.add_patch(box)
+
             # Indicate type of ControlFlowOp and if expression used, print below
             self._ax.text(
                 xpos - x_shift - op_spacer,

releasenotes/notes/fix-default-case-empty-0076264db04d6c7a.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixed an issue in the ``mpl`` circuit drawer where the text would print beyond
+    the end of the box for a :class:`.SwitchCaseOp` if the default case was empty.

test/visual/mpl/circuit/test_circuit_matplotlib_drawer.py

@@ -2044,6 +2044,34 @@ def test_switch_case_op_1_qarg(self):
         )
         self.assertGreaterEqual(ratio, 0.9999)
 
+    def test_switch_case_op_empty_default(self):
+        """Test the SwitchCaseOp with empty default case"""
+        qreg = QuantumRegister(3, "q")
+        creg = ClassicalRegister(3, "cr")
+        circuit = QuantumCircuit(qreg, creg)
+
+        circuit.h([0, 1, 2])
+        circuit.measure([0, 1, 2], [0, 1, 2])
+
+        with circuit.switch(creg) as case:
+            with case(0, 1, 2):
+                circuit.x(0)
+            with case(case.DEFAULT):
+                pass
+        circuit.h(0)
+
+        fname = "switch_case_empty_default.png"
+        self.circuit_drawer(circuit, output="mpl", cregbundle=False, filename=fname)
+
+        ratio = VisualTestUtilities._save_diff(
+            self._image_path(fname),
+            self._reference_path(fname),
+            fname,
+            FAILURE_DIFF_DIR,
+            FAILURE_PREFIX,
+        )
+        self.assertGreaterEqual(ratio, 0.9999)
+
     def test_if_with_expression(self):
         """Test the IfElseOp with an expression"""
         qr = QuantumRegister(3, "qr")