Move displaced line of top-anchored scroll region into scroll-back

terminal/bundles/org.eclipse.terminal.control/src/org/eclipse/terminal/internal/emulator/VT100EmulatorBackend.java

@@ -729,7 +729,11 @@ private int doLineWrap() {
 	private void doNewline() {
 		fWrapPending = false; // DEC STD 070: any move of the cursor forgets a pending wrap
 		if (fCursorLine == fScrollRegion.getBottomLine()) {
-			scrollUp(1);
+			if (fScrollRegion.getTopLine() == 0) {
+				scrollTopAnchoredRegionUpIntoHistory();
+			} else {
+				scrollUp(1);
+			}
 		} else if (fCursorLine + 1 >= fLines) {
 			int h = fTerminal.getHeight();
 			fTerminal.addLine();
@@ -741,6 +745,28 @@ private void doNewline() {
 		}
 	}
 
+	/**
+	 * Scrolls a scroll region whose top margin is the top of the screen: the displaced top line
+	 * is moved into the scroll-back history (like xterm/VTE) instead of being discarded.
+	 * MUST be called from a synchronized block!
+	 */
+	private void scrollTopAnchoredRegionUpIntoHistory() {
+		int regionBottom = fScrollRegion.getBottomLine();
+		if (regionBottom == fLines - 1) {
+			fTerminal.addLine();
+		} else if (fTerminal.getMaxHeight() > 0 && fTerminal.getHeight() < fTerminal.getMaxHeight()) {
+			// the history grows, so the former top line stays where it is and becomes history;
+			// shift the lines below the region back down, vacating the bottom line of the region
+			fTerminal.addLine();
+			fTerminal.scroll(toAbsoluteLine(regionBottom), fLines - regionBottom, 1);
+		} else {
+			// one negative shift that snapshots can track, so selections move with the text
+			fTerminal.scroll(0, toAbsoluteLine(regionBottom) + 1, -1);
+		}
+		// the absolute line of the cursor may have changed
+		setCursorLine(fCursorLine);
+	}
+
 	@Override
 	public void processNewline() {
 		synchronized (fTerminal) {

terminal/bundles/org.eclipse.terminal.control/src/org/eclipse/terminal/internal/model/TerminalTextDataFastScroll.java

@@ -186,6 +186,14 @@ public void scroll(int startLine, int size, int shift) {
 			} else {
 				cleanLines(startLine, Math.min(shift, getHeight() - startLine));
 			}
+		} else if (startLine == 0 && shift < 0 && fHeight - size < size + shift && fHeight == fMaxHeight) {
+			// rotate and copy back only the fewer lines below the region; on a ring with spare
+			// slots a later addLine() would expose the rotated-out lines
+			moveOffset(-shift);
+			for (int i = fHeight - 1; i >= size; i--) {
+				fData.copyLine(fData, getPositionOfLine(i + shift), getPositionOfLine(i));
+			}
+			cleanLines(size + shift, -shift);
 		} else {
 			// we have to copy the lines.
 			if (shift < 0) {

terminal/tests/org.eclipse.terminal.test/src/org/eclipse/terminal/internal/emulator/VT100EmulatorBackendTest.java

@@ -31,6 +31,7 @@
 import org.eclipse.terminal.internal.model.TerminalTextTestHelper;
 import org.eclipse.terminal.model.ITerminalTextData;
 import org.eclipse.terminal.model.ITerminalTextDataReadOnly;
+import org.eclipse.terminal.model.ITerminalTextDataSnapshot;
 import org.eclipse.terminal.model.TerminalStyle;
 import org.junit.jupiter.api.Test;
 
@@ -1006,12 +1007,7 @@ public void testEraseCharacters() {
 	}
 
 	// ---------------------------------------------------------------------------------------------
-	// Characterization tests for the scrolling behavior.
-	//
-	// These lock the current behavior before any change to how scrolling interacts with the
-	// scroll-back history. The regression guards below MUST keep passing through such a change; the
-	// "currentlyDiscards" test documents the behavior that a future fix for top-anchored scroll
-	// regions (eclipse.platform issue 2680) is expected to change.
+	// Tests for how scrolling interacts with the scroll-back history.
 	// ---------------------------------------------------------------------------------------------
 
 	/**
@@ -1088,14 +1084,11 @@ public void testReverseLineFeedAtTopOfRegionScrollsDown() {
 	}
 
 	/**
-	 * Characterization of the <em>current</em> behavior for a scroll region anchored at the top of
-	 * the screen: a newline at the region bottom scrolls in place and discards the top line, creating
-	 * no history. This is the behavior the planned fix for eclipse.platform issue 2680 will change so
-	 * that the discarded line is added to the scroll-back instead. Until then this documents the
-	 * status quo.
+	 * A newline at the bottom of a top-anchored scroll region moves the displaced top line into
+	 * the scroll-back history instead of discarding it.
 	 */
 	@Test
-	public void testNewlineInTopAnchoredScrollRegionCurrentlyDiscardsTopLine() {
+	public void testNewlineInTopAnchoredScrollRegionMovesTopLineToHistory() {
 		ITerminalTextData term = makeITerminalTextData();
 		IVT100EmulatorBackend vt100 = makeBakend(term);
 		term.setMaxHeight(10);
@@ -1106,12 +1099,131 @@ public void testNewlineInTopAnchoredScrollRegionCurrentlyDiscardsTopLine() {
 		vt100.setCursorLine(3);
 		vt100.processNewline();
 
-		assertEquals(5, term.getHeight()); // currently no history is created
-		assertEquals("1111", new String(term.getChars(0))); // "0000" was discarded (lost)
+		assertEquals(6, term.getHeight()); // one line of history was created
+		assertEquals("0000", new String(term.getChars(0))); // ... holding the displaced top line
+		// the screen is now lines 1..5 of the model
+		assertEquals("1111", new String(term.getChars(1)));
+		assertEquals("2222", new String(term.getChars(2)));
+		assertEquals("3333", new String(term.getChars(3)));
+		assertNull(term.getChars(4)); // vacated line at the bottom of the region
+		assertEquals("4444", new String(term.getChars(5))); // footer below the region is untouched
+		assertEquals(3, vt100.getCursorLine()); // cursor stays at the bottom of the region
+	}
+
+	/**
+	 * The same, once the scroll-back is full: the model no longer grows, so the oldest history line
+	 * is dropped instead. This is the steady state of a long running full screen application.
+	 */
+	@Test
+	public void testNewlineInTopAnchoredScrollRegionWithFullHistoryDropsOldestLine() {
+		ITerminalTextData term = makeITerminalTextData();
+		IVT100EmulatorBackend vt100 = makeBakend(term);
+		term.setMaxHeight(6);
+		vt100.setDimensions(5, 4);
+		fill(term, "hhhh\n" + "0000\n" + "1111\n" + "2222\n" + "3333\n" + "4444");
+		assertEquals(6, term.getHeight()); // one line of history, five lines of screen
+
+		vt100.setScrollRegion(0, 3);
+		vt100.setCursorLine(3);
+		vt100.processNewline();
+
+		assertEquals(6, term.getHeight()); // capped, so "hhhh" fell out of the history
+		assertEquals("0000", new String(term.getChars(0))); // the displaced top line took its place
+		assertEquals("1111", new String(term.getChars(1)));
+		assertEquals("2222", new String(term.getChars(2)));
+		assertEquals("3333", new String(term.getChars(3)));
+		assertNull(term.getChars(4)); // vacated line at the bottom of the region
+		assertEquals("4444", new String(term.getChars(5))); // footer below the region is untouched
+		assertEquals(3, vt100.getCursorLine());
+	}
+
+	/**
+	 * With a full history the scroll must reach snapshots as one upward shift of history and region,
+	 * otherwise selections in the region are not moved along with the text.
+	 */
+	@Test
+	public void testNewlineInTopAnchoredScrollRegionWithFullHistoryNotifiesScrollToSnapshots() {
+		ITerminalTextData term = new TerminalTextData();
+		IVT100EmulatorBackend vt100 = makeBakend(term);
+		term.setMaxHeight(6);
+		vt100.setDimensions(5, 4);
+		fill(term, "hhhh\n" + "0000\n" + "1111\n" + "2222\n" + "3333\n" + "4444");
+		ITerminalTextDataSnapshot snapshot = term.makeSnapshot();
+		snapshot.updateSnapshot(true);
+
+		vt100.setScrollRegion(0, 3);
+		vt100.setCursorLine(3);
+		vt100.processNewline();
+		snapshot.updateSnapshot(true);
+
+		assertEquals(0, snapshot.getScrollWindowStartLine());
+		assertEquals(5, snapshot.getScrollWindowSize()); // history and region, not the footer
+		assertEquals(-1, snapshot.getScrollWindowShift());
+		assertEquals("0000", new String(snapshot.getChars(0)));
+		assertEquals("1111", new String(snapshot.getChars(1)));
+		assertEquals("2222", new String(snapshot.getChars(2)));
+		assertEquals("3333", new String(snapshot.getChars(3)));
+		assertNull(snapshot.getChars(4));
+		assertEquals("4444", new String(snapshot.getChars(5)));
+	}
+
+	/**
+	 * A footer of several lines stays in place, both while the history grows and once it is full.
+	 */
+	@Test
+	public void testNewlineInTopAnchoredScrollRegionWithTallFooter() {
+		ITerminalTextData term = new TerminalTextData();
+		IVT100EmulatorBackend vt100 = makeBakend(term);
+		term.setMaxHeight(6);
+		vt100.setDimensions(5, 4);
+		fill(term, "0000\n" + "1111\n" + "2222\n" + "3333\n" + "4444");
+		vt100.setScrollRegion(0, 2);
+		vt100.setCursorLine(2);
+
+		vt100.processNewline(); // the history grows
+		assertEquals(6, term.getHeight());
+		assertEquals("0000", new String(term.getChars(0)));
+		assertEquals("1111", new String(term.getChars(1)));
+		assertEquals("2222", new String(term.getChars(2)));
+		assertNull(term.getChars(3));
+		assertEquals("3333", new String(term.getChars(4)));
+		assertEquals("4444", new String(term.getChars(5)));
+
+		vt100.processNewline(); // the history is full
+		assertEquals(6, term.getHeight());
+		assertEquals("1111", new String(term.getChars(0)));
 		assertEquals("2222", new String(term.getChars(1)));
-		assertEquals("3333", new String(term.getChars(2)));
+		assertNull(term.getChars(2));
 		assertNull(term.getChars(3));
-		assertEquals("4444", new String(term.getChars(4))); // footer below the region is untouched
+		assertEquals("3333", new String(term.getChars(4)));
+		assertEquals("4444", new String(term.getChars(5)));
+		assertEquals(2, vt100.getCursorLine());
+	}
+
+	/**
+	 * A top-anchored scroll region that covers the whole screen has no fixed lines below it, so the
+	 * screen just moves up by one line.
+	 */
+	@Test
+	public void testNewlineInTopAnchoredScrollRegionCoveringWholeScreen() {
+		ITerminalTextData term = makeITerminalTextData();
+		IVT100EmulatorBackend vt100 = makeBakend(term);
+		term.setMaxHeight(10);
+		vt100.setDimensions(5, 4);
+		fill(term, "0000\n" + "1111\n" + "2222\n" + "3333\n" + "4444");
+
+		vt100.setScrollRegion(0, 4); // the region is the whole screen
+		vt100.setCursorLine(4);
+		vt100.processNewline();
+
+		assertEquals(6, term.getHeight());
+		assertEquals("0000", new String(term.getChars(0))); // displaced top line
+		assertEquals("1111", new String(term.getChars(1)));
+		assertEquals("2222", new String(term.getChars(2)));
+		assertEquals("3333", new String(term.getChars(3)));
+		assertEquals("4444", new String(term.getChars(4)));
+		assertNull(term.getChars(5)); // new blank line at the bottom of the screen
+		assertEquals(4, vt100.getCursorLine());
 	}
 
 	@Test

terminal/tests/org.eclipse.terminal.test/src/org/eclipse/terminal/internal/model/AbstractITerminalTextDataTest.java

@@ -573,6 +573,11 @@ public void testScrollNegative() {
 		scrollTest(1, 2, -1, "012345", "02 345");
 		scrollTest(5, 1, -1, "012345", "01234 ");
 		scrollTest(5, 1, -1, "012345", "01234 ");
+		scrollTest(0, 5, -1, "012345", "1234 5");
+		scrollTest(0, 5, -2, "012345", "234  5");
+		scrollTest(0, 4, -1, "012345", "123 45");
+		scrollTest(0, 4, -3, "012345", "3   45");
+		scrollTest(0, 3, -1, "012345", "12 345");
 	}
 
 	@Test

terminal/tests/org.eclipse.terminal.test/src/org/eclipse/terminal/internal/model/TerminalTextDataFastScrollTest.java

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2007, 2018 Wind River Systems, Inc. and others.
+ * Copyright (c) 2007, 2026 Wind River Systems, Inc. and others.
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License 2.0
  * which accompanies this distribution, and is available at
@@ -10,12 +10,48 @@
  *******************************************************************************/
 package org.eclipse.terminal.internal.model;
 
+import static org.eclipse.terminal.internal.model.TerminalTextTestHelper.fillSimple;
+import static org.eclipse.terminal.internal.model.TerminalTextTestHelper.toSimple;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
 import org.eclipse.terminal.model.ITerminalTextData;
+import org.junit.jupiter.api.Test;
 
 public class TerminalTextDataFastScrollTest extends AbstractITerminalTextDataTest {
 	@Override
 	protected ITerminalTextData makeITerminalTextData() {
 		return new TerminalTextDataFastScroll(3);
 	}
 
+	/**
+	 * Scrolling a top-anchored region while the ring has unused capacity must not leave scrolled
+	 * out text where a later {@link ITerminalTextData#addLine()} grows into.
+	 */
+	@Test
+	public void testScrollTopAnchoredRegionThenGrow() {
+		ITerminalTextData term = new TerminalTextDataFastScroll(7);
+		fillSimple(term, "012345");
+		term.scroll(0, 5, -1);
+		term.addLine();
+		assertEquals("1234 5 ", toSimple(term));
+	}
+
+	/**
+	 * Repeated top-anchored scrolls on a full ring, so the rotation also runs from a non-zero
+	 * offset.
+	 */
+	@Test
+	public void testScrollTopAnchoredRegionRepeatedly() {
+		ITerminalTextData term = new TerminalTextDataFastScroll(6);
+		fillSimple(term, "012345");
+		term.scroll(0, 6, -2);
+		assertEquals("2345  ", toSimple(term));
+		term.setChar(4, 0, 'a', null);
+		term.setChar(5, 0, 'b', null);
+		term.scroll(0, 5, -1);
+		assertEquals("345a b", toSimple(term));
+		term.scroll(0, 5, -1);
+		assertEquals("45a  b", toSimple(term));
+	}
+
 }