Follow console output only while scrolled to the bottom

The combined view never scrolled as text arrived, and the other consoles
always jumped to the end, even when the user had scrolled upward.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Author: ctrueden

src/main/java/org/scijava/ui/swing/script/ConsoleArea.java

@@ -32,12 +32,15 @@
 
 import java.awt.Color;
 import java.awt.Graphics;
+import java.awt.Rectangle;
 import java.util.List;
 import java.util.concurrent.CopyOnWriteArrayList;
 
 import javax.swing.event.DocumentEvent;
 import javax.swing.event.DocumentListener;
 import javax.swing.JTextArea;
+import javax.swing.JViewport;
+import javax.swing.SwingUtilities;
 import javax.swing.plaf.basic.BasicTextAreaUI;
 import javax.swing.text.BadLocationException;
 import javax.swing.text.Document;
@@ -127,10 +130,30 @@ public void append(final String text, final boolean error) {
 		if (text == null || text.isEmpty()) return;
 		synchronized (appendLock) {
 			chunks.add(new Mark(getDocument().getLength(), error));
-			append(text);
+			appendFollowing(this, text);
 		}
 	}
 
+	/**
+	 * Appends text to the given area, then scrolls to the end of the text if the
+	 * area was already scrolled to the bottom (or is not currently displayed).
+	 * If the user has scrolled upward, the view is left alone.
+	 */
+	public static void appendFollowing(final JTextArea area, final String text) {
+		final boolean follow = isAtBottom(area);
+		area.append(text);
+		if (follow) {
+			SwingUtilities.invokeLater(() -> area.setCaretPosition(area.getDocument().getLength()));
+		}
+	}
+
+	private static boolean isAtBottom(final JTextArea area) {
+		if (!(area.getParent() instanceof JViewport)) return true;
+		final Rectangle visible = area.getVisibleRect();
+		// Note: allow a little slack for rounding and partially visible lines.
+		return visible.y + visible.height >= area.getHeight() - 2;
+	}
+
 	/** Sets foreground, background and caret colors together. */
 	public void setColors(final Color fg, final Color bg) {
 		setForeground(fg);

src/main/java/org/scijava/ui/swing/script/JTextAreaOutputStream.java

@@ -104,8 +104,7 @@ public void flushQueue() {
 			catch (final BadLocationException e) {
 				e.printStackTrace();
 			}
-			textArea.append(sb.toString());
-			textArea.setCaretPosition(textArea.getDocument().getLength());
+			ConsoleArea.appendFollowing(textArea, sb.toString());
 		}
 	}
 

src/main/java/org/scijava/ui/swing/script/JTextAreaWriter.java

@@ -114,8 +114,7 @@ public void flushQueue() {
 			catch (final BadLocationException e) {
 				log.error(e);
 			}
-			textArea.append(sb.toString());
-			textArea.setCaretPosition(textArea.getDocument().getLength());
+			ConsoleArea.appendFollowing(textArea, sb.toString());
 		}
 	}