Allow selecting loops in the UI.

css/style.css

@@ -571,6 +571,10 @@ select {
   justify-content: space-between;
 }
 
+#multi-cell-constraint-loop-container {
+  margin-bottom: -10px;
+}
+
 .description {
   line-height: 1;
   font-size: 12px;

index.html

@@ -224,6 +224,10 @@ <h2>Create from selection</h2>
               </div>
               <div id="multi-cell-constraint-description" class="description">
               </div>
+              <div id="multi-cell-constraint-loop-container">
+                <input type="checkbox" name="is-loop" id="multi-cell-constraint-loop-input"></input>
+                <label for="multi-cell-constraint-loop-input">Loop</label>
+              </div>
               <div>
                 <div id="multi-cell-constraint-value-container"></div>
                 <button type="submit" name="add-constraint" title="Add constraint" disabled>

js/render_page.js

@@ -1105,6 +1105,9 @@ class ConstraintManager {
     const valueContainer = document.getElementById('multi-cell-constraint-value-container');
     const valueElems = [];
 
+    const loopContainer = document.getElementById('multi-cell-constraint-loop-container');
+    loopContainer.style.display = 'none';
+
     // Initialize defaults.
     for (const [name, config] of Object.entries(this._multiCellConstraints)) {
       config.text ||= name;
@@ -1158,6 +1161,12 @@ class ConstraintManager {
         valueContainer.style.visibility = 'hidden';
       }
 
+      if (config.constraintClass.LOOPS_ALLOWED) {
+        loopContainer.style.display = 'block';
+      } else {
+        loopContainer.style.display = 'none';
+      }
+
       descriptionElem.textContent = config.description;
 
       if (!selectionForm.classList.contains('disabled')) {
@@ -1182,6 +1191,10 @@ class ConstraintManager {
     if (!config) throw ('Unknown constraint type: ' + type);
     if (config.elem.disabled) throw ('Invalid selection for ' + type);
 
+    if (config.constraintClass.LOOPS_ALLOWED && formData.get('is-loop')) {
+      cells.push('LOOP');
+    }
+
     if (config.constraintClass === SudokuConstraint.Quad) {
       const valuesStr = formData.get(type + '-value');
       const values = valuesStr.split(/[, ]+/).map(v => +v).filter(