merge: do not override user modifications when resolving non-conflicting changes

platform/diff-impl/src/com/intellij/diff/merge/TextMergeViewer.java

@@ -878,7 +878,8 @@ public class TextMergeViewer implements MergeTool.MergeViewer {
         }
         else {
           return !change.isResolved() &&
-                 change.isChange(side);
+                 change.isChange(side) &&
+                 !isChangeRangeModified(change);
         }
       });
     }
@@ -903,6 +904,7 @@ public class TextMergeViewer implements MergeTool.MergeViewer {
       else {
         if (change.isResolved(side)) return;
         if (!change.isChange(side)) return;
+        if (isChangeRangeModified(change)) return;
         Side masterSide = side.select(Side.LEFT,
                                       change.isChange(Side.LEFT) ? Side.LEFT : Side.RIGHT,
                                       Side.RIGHT);
@@ -1157,7 +1159,8 @@ public class TextMergeViewer implements MergeTool.MergeViewer {
           return canResolveConflictedChange(change);
         }
         else {
-          return !change.isResolved();
+          return !change.isResolved() &&
+                 !isChangeRangeModified(change);
         }
       }
 

platform/diff-impl/tests/com/intellij/diff/merge/MergeTest.kt

@@ -386,6 +386,53 @@ class MergeTest : MergeTestBase() {
 
       assertContent("y z_Y_y")
     }
+
+    test1("y z", "x y z", "x y") {
+      assertTrue(0.canResolveConflict())
+
+      replaceText(2, 3, "U")
+      assertContent("x U z")
+
+      assertFalse(0.canResolveConflict())
+    }
+
+    test2("y z_Y_x y", "x y z_Y_x y z", "x y_Y_y z") {
+      assertTrue(0.canResolveConflict())
+      assertTrue(1.canResolveConflict())
+
+
+      replaceText(2, 3, "U")
+
+      assertFalse(0.canResolveConflict())
+      assertTrue(1.canResolveConflict())
+      assertContent("x U z_Y_x y z")
+
+
+      checkUndo(1) {
+        1.resolve()
+      }
+      1.assertResolved(BOTH)
+      1.assertContent("y")
+
+      assertFalse(0.canResolveConflict())
+      assertFalse(1.canResolveConflict())
+      assertContent("x U z_Y_y")
+
+
+      replaceText(2, 3, "y")
+
+      assertTrue(0.canResolveConflict())
+      assertFalse(1.canResolveConflict())
+
+
+      checkUndo(1) {
+        0.resolve()
+      }
+
+      assertFalse(0.canResolveConflict())
+      assertFalse(1.canResolveConflict())
+      assertContent("y_Y_y")
+    }
   }
 
   fun testUndoSimple() {
@@ -634,5 +681,50 @@ class MergeTest : MergeTestBase() {
                     delete modify
                     7 ======""".trimIndent())
     }
+
+    testN(text1, text2, text3) {
+      replaceText(!5 - 0, !5 - 0, "USER ")
+
+      checkUndo(1) {
+        runApplyNonConflictsAction(ThreeSide.BASE)
+      }
+
+      assertChangesCount(2)
+      assertContent("""
+                    1 ======
+                    insert left
+                    2 ======
+                    3 ======
+                    new both
+                    4 ======
+                    USER modify
+                    5 ======
+                    modify right
+                    6 ======
+                    delete modify
+                    7 ======""".trimIndent())
+    }
+
+    testN(text1, text2, text3) {
+      replaceText(!7 - 0, !7 - 0, "USER ")
+
+      checkUndo(1) {
+        runApplyNonConflictsAction(ThreeSide.RIGHT)
+      }
+
+      assertChangesCount(3)
+      assertContent("""
+                    1 ======
+                    2 ======
+                    3 ======
+                    new both
+                    4 ======
+                    modify both
+                    5 ======
+                    USER modify
+                    6 ======
+                    delete modify
+                    7 ======""".trimIndent())
+    }
   }
 }

platform/diff-impl/tests/com/intellij/diff/merge/MergeTestBase.kt

@@ -169,6 +169,11 @@ abstract class MergeTestBase : DiffTestCase() {
       }
     }
 
+    fun Int.canResolveConflict(): Boolean {
+      val change = change(this)
+      return viewer.canResolveConflictedChange(change)
+    }
+
     //
     // Text modification
     //