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

@@ -354,7 +354,11 @@ private int doLineWrap() {
 	 */
 	private void doNewline() {
 		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();
@@ -366,6 +370,26 @@ 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() {
+		// adding a line moves the whole screen window up by one line: the former top line of the
+		// screen (and of the region) becomes the newest scroll-back line and the new bottom line of
+		// the screen is blank
+		fTerminal.addLine();
+		int regionBottom = fScrollRegion.getBottomLine();
+		if (regionBottom < fLines - 1) {
+			// lines below the region must not move: shift them back down by one line, which also
+			// vacates the new bottom line of the region
+			fTerminal.scroll(toAbsoluteLine(regionBottom), fLines - regionBottom, 1);
+		}
+		// the absolute line of the cursor may have changed
+		setCursorLine(fCursorLine);
+	}
+
 	@Override
 	public void processNewline() {
 		synchronized (fTerminal) {

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

@@ -979,12 +979,7 @@ 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.
 	// ---------------------------------------------------------------------------------------------
 
 	/**
@@ -1061,14 +1056,11 @@ 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);
@@ -1079,11 +1071,14 @@ 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("2222", new String(term.getChars(1)));
-		assertEquals("3333", new String(term.getChars(2)));
-		assertNull(term.getChars(3));
-		assertEquals("4444", new String(term.getChars(4))); // footer below the region is untouched
+		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
 	}
 }