Merge pull request #1133 from ckeditor/t/1132

Fix: Fixed a bug in `Range#getTransformedByDelta()` that caused editor to crash after some `MergeDelta`s were transformed. Closes #1132.

Author: Reinmar

src/model/range.js

@@ -492,8 +492,16 @@ export default class Range {
 				// Collapsed range is in merged element, at the beginning or at the end of it.
 				// Without fix, the range would end up in the graveyard, together with removed element.
 				// <p>foo</p><p>[]bar</p> -> <p>foobar</p><p>[]</p> -> <p>foobar</p> -> <p>foo[]bar</p>
-				// <p>foo</p><p>bar[]</p>
-				return [ new Range( targetPosition.getShiftedBy( this.start.offset ) ) ];
+				// <p>foo</p><p>bar[]</p> -> <p>foobar</p><p>[]</p> -> <p>foobar</p> -> <p>foobar[]</p>
+				//
+				// In most cases, `sourceRange.start.offset` for merge delta's move operation would be 0,
+				// so this formula might look overcomplicated.
+				// However in some scenarios, after operational transformation, move operation might not
+				// in fact start from 0 and we need to properly count new offset.
+				// https://github.com/ckeditor/ckeditor5-engine/pull/1133#issuecomment-329080668.
+				const offset = this.start.offset - sourceRange.start.offset;
+
+				return [ new Range( targetPosition.getShiftedBy( offset ) ) ];
 			}
 			//
 			// Other edge cases:

tests/model/range.js

@@ -9,6 +9,9 @@ import Element from '../../src/model/element';
 import Text from '../../src/model/text';
 import Document from '../../src/model/document';
 import TreeWalker from '../../src/model/treewalker';
+import MergeDelta from '../../src/model/delta/mergedelta';
+import MoveOperation from '../../src/model/operation/moveoperation';
+import RemoveOperation from '../../src/model/operation/removeoperation';
 import CKEditorError from '@ckeditor/ckeditor5-utils/src/ckeditorerror';
 import { jsonParseStringify } from '../../tests/model/_utils/utils';
 
@@ -1056,6 +1059,35 @@ describe( 'Range', () => {
 				expect( transformed[ 0 ].start.path ).to.deep.equal( [ 0, 0, 1 ] );
 				expect( transformed[ 0 ].end.path ).to.deep.equal( [ 0, 1, 1 ] );
 			} );
+
+			// #1132.
+			it( 'merge delta has move operation that does not start from offset 0', () => {
+				// This scenario is a test for a rare situation, that after some OT, a move operation in
+				// merge delta does not start from 0 offset.
+				//
+				// It happens that move operation in merge delta becomes "do nothing move operation", something like:
+				//
+				// move range [ a, x ] - [ a, y ] to [ a, x ]
+				// for example: move [ 0, 3 ] - [ 0, 6 ] -> [ 0, 3 ]
+				//
+				// This is a result of valid transformation and we need to check if range is properly transformed
+				// when such unusual delta is generated.
+				// For more see: https://github.com/ckeditor/ckeditor5-engine/pull/1133#issuecomment-329080668.
+				//
+				// For this test scenario assume: <p>foobar[]</p>, "bar" is moved between "o" and "b".
+				// Expect state after transformation is that nothing has changed.
+				const range = new Range( new Position( root, [ 0, 6 ] ), new Position( root, [ 0, 6 ] ) );
+
+				const delta = new MergeDelta();
+				delta.addOperation( new MoveOperation( new Position( root, [ 0, 3 ] ), 3, new Position( root, [ 0, 3 ] ), 0 ) );
+				delta.addOperation( new RemoveOperation( new Position( root, [ 1 ] ), 1, new Position( doc.graveyard, [ 0 ] ), 1 ) );
+
+				const transformed = range.getTransformedByDelta( delta );
+
+				expect( transformed.length ).to.equal( 1 );
+				expect( transformed[ 0 ].start.path ).to.deep.equal( [ 0, 6 ] );
+				expect( transformed[ 0 ].end.path ).to.deep.equal( [ 0, 6 ] );
+			} );
 		} );
 
 		describe( 'by WrapDelta', () => {