fix - chat not scrolling to the newest messages in long conversations

com.microsoft.copilot.eclipse.ui.test/src/com/microsoft/copilot/eclipse/ui/chat/ChatContentViewerTest.java

@@ -9,14 +9,17 @@
 import static org.mockito.Mockito.when;
 
 import java.lang.reflect.Field;
+import java.lang.reflect.Method;
 import java.util.List;
 import java.util.Map;
 
 import org.eclipse.lsp4j.WorkDoneProgressKind;
 import org.eclipse.swt.SWT;
 import org.eclipse.swt.widgets.Display;
+import org.eclipse.swt.widgets.ScrollBar;
 import org.eclipse.swt.widgets.Shell;
 import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
@@ -104,6 +107,118 @@ private Map<String, BaseTurnWidget> getTurnsMap(ChatContentViewer viewer) {
     return (Map<String, BaseTurnWidget>) getFieldValue(viewer, "turns");
   }
 
+  @Test
+  void testClampOffset_clampsToScrollableRange() {
+    SwtUtils.invokeOnDisplayThread(() -> {
+      sizeViewer(400, 300);
+      setFieldValue(viewer, "totalHeight", 1000);
+      int maxOffset = invokeIntMethod(viewer, "maxOffset");
+
+      Assertions.assertTrue(maxOffset > 0, "content taller than the viewport should be scrollable");
+      Assertions.assertEquals(0, invokeIntMethod(viewer, "clampOffset", -50),
+          "negative offset should clamp to 0");
+      Assertions.assertEquals(maxOffset, invokeIntMethod(viewer, "clampOffset", 5000),
+          "offset past the end should clamp to maxOffset");
+      Assertions.assertEquals(200, invokeIntMethod(viewer, "clampOffset", 200),
+          "in-range offset should be left unchanged");
+    });
+  }
+
+  @Test
+  void testMaxOffset_isZeroWhenContentFitsViewport() {
+    SwtUtils.invokeOnDisplayThread(() -> {
+      sizeViewer(400, 300);
+      setFieldValue(viewer, "totalHeight", 200);
+
+      Assertions.assertEquals(0, invokeIntMethod(viewer, "maxOffset"),
+          "content shorter than the viewport should not be scrollable");
+    });
+  }
+
+  @Test
+  void testUpdateScrollBar_rangeMatchesTotalHeight() {
+    SwtUtils.invokeOnDisplayThread(() -> {
+      sizeViewer(400, 300);
+      setFieldValue(viewer, "totalHeight", 1000);
+      setFieldValue(viewer, "scrollOffset", 120);
+
+      invokeVoidMethod(viewer, "updateScrollBar", 300);
+
+      ScrollBar bar = viewer.getVerticalBar();
+      Assertions.assertTrue(bar.getEnabled(), "scrollbar should be enabled when content overflows");
+      Assertions.assertEquals(1000, bar.getMaximum(), "scrollbar maximum should equal totalHeight");
+      Assertions.assertEquals(300, bar.getThumb(), "scrollbar thumb should equal the viewport height");
+      Assertions.assertEquals(120, bar.getSelection(), "scrollbar selection should equal scrollOffset");
+    });
+  }
+
+  @Test
+  void testUpdateScrollBar_disabledWhenContentFits() {
+    SwtUtils.invokeOnDisplayThread(() -> {
+      sizeViewer(400, 300);
+      setFieldValue(viewer, "totalHeight", 200);
+
+      invokeVoidMethod(viewer, "updateScrollBar", 300);
+
+      Assertions.assertFalse(viewer.getVerticalBar().getEnabled(),
+          "scrollbar should be disabled when all content fits");
+    });
+  }
+
+  @Test
+  void testIsViewportAtBottom_togglesWhenLeavingAndReturning() {
+    SwtUtils.invokeOnDisplayThread(() -> {
+      sizeViewer(400, 300);
+      setFieldValue(viewer, "totalHeight", 1000);
+      int maxOffset = invokeIntMethod(viewer, "maxOffset");
+
+      setFieldValue(viewer, "scrollOffset", maxOffset);
+      Assertions.assertTrue(invokeBooleanMethod(viewer, "isViewportAtBottom"),
+          "pinned to the very bottom should report at-bottom");
+
+      setFieldValue(viewer, "scrollOffset", 0);
+      Assertions.assertFalse(invokeBooleanMethod(viewer, "isViewportAtBottom"),
+          "scrolled up to the top should report not-at-bottom");
+
+      // Just inside the bottom threshold (SCROLL_THRESHOLD = 100) should count as at-bottom again.
+      setFieldValue(viewer, "scrollOffset", maxOffset - 50);
+      Assertions.assertTrue(invokeBooleanMethod(viewer, "isViewportAtBottom"),
+          "back within the bottom threshold should report at-bottom again");
+    });
+  }
+
+  /** Gives the viewer (and its content child) a deterministic client area for scroll-model math. */
+  private void sizeViewer(int width, int height) {
+    viewer.setSize(width, height);
+    viewer.layout(true, true);
+  }
+
+  private int invokeIntMethod(Object target, String name, int arg) {
+    return (int) invokeMethod(target, name, new Class<?>[] {int.class}, arg);
+  }
+
+  private int invokeIntMethod(Object target, String name) {
+    return (int) invokeMethod(target, name, new Class<?>[] {});
+  }
+
+  private boolean invokeBooleanMethod(Object target, String name) {
+    return (boolean) invokeMethod(target, name, new Class<?>[] {});
+  }
+
+  private void invokeVoidMethod(Object target, String name, int arg) {
+    invokeMethod(target, name, new Class<?>[] {int.class}, arg);
+  }
+
+  private Object invokeMethod(Object target, String name, Class<?>[] paramTypes, Object... args) {
+    try {
+      Method method = target.getClass().getDeclaredMethod(name, paramTypes);
+      method.setAccessible(true);
+      return method.invoke(target, args);
+    } catch (Exception e) {
+      throw new RuntimeException("Failed to invoke method " + name, e);
+    }
+  }
+
   private Object getFieldValue(Object target, String fieldName) {
     try {
       Field field = target.getClass().getDeclaredField(fieldName);