fix: minor bugfix to ToggleLineCommentHandler.

As written, the removeLineComments() algorithm expects the commented
lines to be processed in order, otherwise the document can become
corrupted as the wrong characters get removed. This fix ensures that the
lines are processed in order.

org.eclipse.tm4e.languageconfiguration.tests/src/main/java/org/eclipse/tm4e/languageconfiguration/tests/TestComment.java

@@ -126,6 +126,36 @@ public void testToggleLineCommentUseBlockCommentAndWindowsEOL() throws Exception
 		assertEquals("a\r\n\r\nb\r\n\r\nc", doc.get());
 		checktTextSelection(editor.getSelectionProvider().getSelection(), 0, 0);
 	}
+
+	@Test
+	public void testToggleLineComment() throws Exception {
+		final var now = System.currentTimeMillis();
+		final var proj = ResourcesPlugin.getWorkspace().getRoot().getProject(getClass().getName() + now);
+		proj.create(null);
+		proj.open(null);
+		final var file = proj.getFile("whatever.lc-test");
+		// smallest padding so that the selected lines would not come out of a HashSet in order.
+		String padding = "0\n1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n11\n12\n13\n";
+		String input = padding + "a\nb\nc\n" + padding;
+		file.create(new ByteArrayInputStream(input.getBytes()), true, null);
+		final var editor = (ITextEditor) IDE.openEditor(
+				PlatformUI.getWorkbench().getActiveWorkbenchWindow().getActivePage(), file,
+				"org.eclipse.ui.genericeditor.GenericEditor");
+		final var doc = editor.getDocumentProvider().getDocument(editor.getEditorInput());
+		final var service = PlatformUI.getWorkbench().getService(IHandlerService.class);
+		String text = doc.get();
+		int indexOfA = text.indexOf('a');
+		int lengthToC = text.indexOf('c') + 1 - indexOfA;
+		editor.getSelectionProvider().setSelection(new TextSelection(indexOfA, lengthToC));
+		service.executeCommand(ToggleLineCommentHandler.TOGGLE_LINE_COMMENT_COMMAND_ID, null);
+		String commented = padding + "//a\n//b\n//c\n" + padding;
+		assertEquals(commented, doc.get());
+
+		// Repeatedly executed toggle comment command should remove the comments inserted previously
+		service.executeCommand(ToggleLineCommentHandler.TOGGLE_LINE_COMMENT_COMMAND_ID, null);
+		assertEquals(input, doc.get());
+		checktTextSelection(editor.getSelectionProvider().getSelection(), indexOfA, lengthToC);
+	}
 
 	@Test
 	public void testToggleBlockCommentUseLineComment() throws Exception {

org.eclipse.tm4e.languageconfiguration/src/main/java/org/eclipse/tm4e/languageconfiguration/internal/ToggleLineCommentHandler.java

@@ -533,7 +533,7 @@ private static void removeLineComments(final IDocument document, final ITextSele
 		// Filter out the blank lines and lines that are outside of the text selection
 		final Set<Integer> lines = computeLines(selection, document).stream()
 				.filter(l -> l >= selection.getStartLine() && l <= selection.getEndLine())
-				.collect(Collectors.toSet());
+				.collect(Collectors.toCollection(TreeSet::new));
 
 		boolean isFirstLineUpdated = false;
 		for (final int lineNumber : lines) {