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

Full-screen CLIs such as Codex in --no-alt-screen mode render through
DECSTBM scroll regions anchored at the top of the screen and scroll by
emitting newlines at the region bottom. VT100EmulatorBackend scrolled
such a region in place and discarded the displaced top line, so no
scroll-back history was created and the output could not be scrolled
backwards.

Like xterm and VTE, a newline at the bottom of a scroll region whose
top margin is the top of the screen now moves the displaced top line
into the scroll-back history, while the lines below the region keep
their position on the screen. Scroll regions with a top margin below
the top of the screen still scroll in place and discard the top line.

The characterization test documenting the old behavior is flipped to
assert that the history is created; the remaining scrolling guards are
unchanged.

#2680

Author: vogella

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 +370,28 @@ private void doNewline() {
 		}
 	}
 
+	/**
+	 * Scrolls a scroll region whose top margin is the top of the screen. Like in xterm or VTE,
+	 * the line displaced at the top of the region is moved into the scroll-back history instead
+	 * of being discarded, while the lines below the region keep their position on the screen
+	 * (https://github.com/eclipse-platform/eclipse.platform/issues/2680).
+	 * 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

@@ -1061,14 +1061,13 @@ 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.
+	 * Fix for eclipse.platform issue 2680: for a scroll region anchored at the top of the screen, a
+	 * newline at the region bottom moves the displaced top line into the scroll-back history (like
+	 * xterm/VTE) instead of discarding it. The lines below the region keep their position on the
+	 * screen.
 	 */
 	@Test
-	public void testNewlineInTopAnchoredScrollRegionCurrentlyDiscardsTopLine() {
+	public void testNewlineInTopAnchoredScrollRegionMovesTopLineToHistory() {
 		ITerminalTextData term = makeITerminalTextData();
 		IVT100EmulatorBackend vt100 = makeBakend(term);
 		term.setMaxHeight(10);
@@ -1079,11 +1078,14 @@ 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
 	}
 }