LT-22710: Make phonological rule formula cells non-editable

Src/LexText/LexTextControls/PatternVcBase.cs

@@ -213,6 +213,25 @@ protected void AddExtraLines(int numLines, int tag, IVwEnv vwenv)
 			}
 		}
 
+		/// <summary>
+		/// Keeps the zero-width-space boundary spans editable even inside a pattern that has
+		/// been marked not editable as a whole.
+		/// </summary>
+		/// <remarks>
+		/// Clicking an item, and every insert and delete, place the cursor through
+		/// MakeTextSelInObj with fEditable true, so a cell has to offer at least one editable
+		/// position or those commands stop working with no error. These boundary spans are the
+		/// position. They are safe to leave editable because ktagLeftBoundary and
+		/// ktagRightBoundary are fake tags rather than model properties, and
+		/// RuleFormulaVcBase.UpdateProp returns the value unchanged, so an edit landing here
+		/// reaches nothing real.
+		/// </remarks>
+		protected static void MarkBoundaryEditable(IVwEnv vwenv)
+		{
+			vwenv.set_IntProperty((int)FwTextPropType.ktptEditable, (int)FwTextPropVar.ktpvEnum,
+				(int)TptEditable.ktptIsEditable);
+		}
+
 		protected void OpenSingleLinePile(IVwEnv vwenv, int maxNumLines)
 		{
 			OpenSingleLinePile(vwenv, maxNumLines, true);
@@ -227,6 +246,7 @@ protected void OpenSingleLinePile(IVwEnv vwenv, int maxNumLines, bool addBoundar
 			if (addBoundary)
 			{
 				vwenv.Props = m_bracketProps;
+				MarkBoundaryEditable(vwenv);
 				vwenv.AddProp(ktagLeftBoundary, this, kfragZeroWidthSpace);
 			}
 		}
@@ -241,6 +261,7 @@ protected void CloseSingleLinePile(IVwEnv vwenv, bool addBoundary)
 			if (addBoundary)
 			{
 				vwenv.Props = m_bracketProps;
+				MarkBoundaryEditable(vwenv);
 				vwenv.AddProp(ktagRightBoundary, this, kfragZeroWidthSpace);
 			}
 			vwenv.CloseParagraph();

Src/LexText/LexTextControls/PatternView.cs

@@ -38,11 +38,11 @@ public PatternEditingHelper(LcmCache cache, IEditingCallbacks callbacks)
 			{
 			}
 
-			public override bool CanCopy()
-			{
-				return false;
-			}
-
+			/// <summary>
+			/// Cut and paste stay refused because they would change the rule. Copy is not
+			/// overridden: a read-only view still has to let the formula be copied out, and the
+			/// base implementation already requires a real range selection.
+			/// </summary>
 			public override bool CanCut()
 			{
 				return false;
@@ -69,6 +69,15 @@ protected override EditingHelper CreateEditingHelper()
 			return new PatternEditingHelper(Cache, this);
 		}
 
+		/// <summary>
+		/// Activate() is disabled by default in ReadOnlyViews, but a pattern editor does want to
+		/// show selections so the user can see what a chooser insert/delete will act on.
+		/// </summary>
+		protected override bool AllowDisplaySelection
+		{
+			get { return true; }
+		}
+
 		public void Init(Mediator mediator, PropertyTable propertyTable, int hvo, IPatternControl patternControl, PatternVcBase vc, int rootFrag, ISilDataAccess sda)
 		{
 			CheckDisposed();

Src/LexText/Morphology/AffixRuleFormulaVc.cs

@@ -140,6 +140,7 @@ public override void Display(IVwEnv vwenv, int hvo, int frag)
 					VwLength tableLen;
 					tableLen.nVal = 10000;
 					tableLen.unit = VwUnit.kunPercent100;
+					MarkFormulaNotEditable(vwenv);
 					vwenv.OpenTable(3, tableLen, 0, VwAlignment.kvaLeft, VwFramePosition.kvfpVoid, VwRule.kvrlNone, 0, 0, false);
 
 					VwLength inputLen;

Src/LexText/Morphology/MetaRuleFormulaVc.cs

@@ -103,6 +103,7 @@ public override void Display(IVwEnv vwenv, int hvo, int frag)
 					VwLength tableLen;
 					tableLen.nVal = 10000;
 					tableLen.unit = VwUnit.kunPercent100;
+					MarkFormulaNotEditable(vwenv);
 					vwenv.OpenTable(5, tableLen, 0, VwAlignment.kvaCenter, VwFramePosition.kvfpVoid, VwRule.kvrlNone, 0, 4000, false);
 
 					VwLength ctxtLen;

Src/LexText/Morphology/MorphologyEditorDllTests/RuleFormulaControlWiringTests.cs

@@ -0,0 +1,46 @@
+// Copyright (c) 2026 SIL International
+// This software is licensed under the LGPL, version 2.1 or later
+// (http://www.gnu.org/licenses/lgpl-2.1.html)
+
+using NUnit.Framework;
+
+namespace SIL.FieldWorks.XWorks.MorphologyEditor
+{
+	/// <summary>
+	/// Constructing each real rule formula control must produce a read-only rootsite, since a
+	/// rule cell is modifiable only by chooser-insert and delete.
+	/// </summary>
+	[TestFixture]
+	public class RuleFormulaControlWiringTests
+	{
+		[Test]
+		public void RegRuleFormulaControl_RootSiteIsReadOnly()
+		{
+			using (var control = new RegRuleFormulaControl(null))
+			{
+				Assert.That(control.RootSite.ReadOnlyView, Is.True,
+					"RegRuleFormulaControl must wire up a read-only rootsite");
+			}
+		}
+
+		[Test]
+		public void MetaRuleFormulaControl_RootSiteIsReadOnly()
+		{
+			using (var control = new MetaRuleFormulaControl(null))
+			{
+				Assert.That(control.RootSite.ReadOnlyView, Is.True,
+					"MetaRuleFormulaControl must wire up a read-only rootsite");
+			}
+		}
+
+		[Test]
+		public void AffixRuleFormulaControl_RootSiteIsReadOnly()
+		{
+			using (var control = new AffixRuleFormulaControl(null))
+			{
+				Assert.That(control.RootSite.ReadOnlyView, Is.True,
+					"AffixRuleFormulaControl must wire up a read-only rootsite");
+			}
+		}
+	}
+}