Preserve frame diffing on full screen redraw

spring-shell-jline/src/main/java/org/springframework/shell/jline/tui/component/view/TerminalUI.java

@@ -60,6 +60,7 @@
  *
  * @author Janne Valkealahti
  * @author Piotr Olaszewski
+ * @author David Pilar
  */
 public class TerminalUI implements ViewService {
 
@@ -269,10 +270,14 @@ private synchronized void display() {
 		requireNonNull(display);
 		requireNonNull(size);
 
-		size.copy(terminal.getSize());
+		Size currentSize = terminal.getSize();
+		boolean sizeChanged = size.getRows() != currentSize.getRows() || size.getColumns() != currentSize.getColumns();
+		size.copy(currentSize);
 		if (fullScreen) {
-			display.clear();
-			display.reset();
+			if (sizeChanged) {
+				display.clear();
+				display.reset();
+			}
 			display.resize(size.getRows(), size.getColumns());
 			Rectangle rect = fullScreenViewRect.apply(terminal, rootView);
 			if (rootView != null) {

spring-shell-jline/src/test/java/org/springframework/shell/jline/tui/component/view/TerminalUiRedrawTests.java

@@ -0,0 +1,112 @@
+/*
+ * Copyright 2023-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.shell.jline.tui.component.view;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.nio.charset.StandardCharsets;
+
+import org.jline.terminal.Size;
+import org.jline.terminal.Terminal;
+import org.jline.terminal.impl.DumbTerminal;
+import org.jline.utils.Display;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.shell.jline.tui.component.view.control.BoxView;
+import org.springframework.test.util.ReflectionTestUtils;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Full screen redraw must preserve JLine frame diffing: an unchanged frame writes nothing
+ * and a full rewrite happens only on resize. Frames are rendered by invoking the private
+ * {@code display()} directly, inspecting the bytes written to the terminal.
+ *
+ * @author David Pilar
+ */
+class TerminalUiRedrawTests {
+
+	private static final String MARKER = "REDRAW-MARKER";
+
+	private static final int ROWS = 24;
+
+	private static final int COLS = 80;
+
+	@Test
+	void fullScreenRedrawWithoutChangesDoesNotRewriteScreen() throws Exception {
+		ByteArrayOutputStream out = new ByteArrayOutputStream();
+		Terminal terminal = dumbTerminal(out, COLS, ROWS);
+		TerminalUI ui = fullScreenUi(terminal);
+
+		int firstFrameBytes = renderFrame(ui, out);
+		assertThat(out.toString(StandardCharsets.UTF_8)).contains(MARKER);
+
+		// identical second frame: nothing changed
+		out.reset();
+		int secondFrameBytes = renderFrame(ui, out);
+
+		assertThat(out.toString(StandardCharsets.UTF_8))
+			.as("an unchanged frame must not re-emit screen content (frame diffing preserved)")
+			.doesNotContain(MARKER);
+		assertThat(secondFrameBytes)
+			.as("an unchanged frame must write far fewer bytes than the initial draw; frame1=%d frame2=%d",
+					firstFrameBytes, secondFrameBytes)
+			.isLessThan(firstFrameBytes / 4);
+	}
+
+	@Test
+	void fullScreenRedrawAfterResizeRewritesScreen() throws Exception {
+		ByteArrayOutputStream out = new ByteArrayOutputStream();
+		Terminal terminal = dumbTerminal(out, COLS, ROWS);
+		TerminalUI ui = fullScreenUi(terminal);
+
+		renderFrame(ui, out);
+		assertThat(out.toString(StandardCharsets.UTF_8)).contains(MARKER);
+
+		// resize: next frame must redraw fully
+		out.reset();
+		terminal.setSize(new Size(COLS + 20, ROWS + 6));
+		renderFrame(ui, out);
+
+		assertThat(out.toString(StandardCharsets.UTF_8)).as("a frame following a resize must redraw the whole screen")
+			.contains(MARKER);
+	}
+
+	private TerminalUI fullScreenUi(Terminal terminal) {
+		TerminalUI ui = new TerminalUI(terminal);
+		BoxView view = new BoxView();
+		view.setShowBorder(true);
+		view.setTitle(MARKER);
+		ui.setRoot(view, true);
+		// private state run()/loop() would set up
+		ReflectionTestUtils.setField(ui, "display", new Display(terminal, true));
+		ReflectionTestUtils.setField(ui, "size", new Size());
+		return ui;
+	}
+
+	private int renderFrame(TerminalUI ui, ByteArrayOutputStream out) {
+		ReflectionTestUtils.invokeMethod(ui, "display");
+		return out.size();
+	}
+
+	private Terminal dumbTerminal(ByteArrayOutputStream out, int cols, int rows) throws Exception {
+		Terminal terminal = new DumbTerminal("terminal", "ansi", new ByteArrayInputStream(new byte[0]), out,
+				StandardCharsets.UTF_8);
+		terminal.setSize(new Size(cols, rows));
+		return terminal;
+	}
+
+}