Have chats respond to PgUp and PgDown

Resolves #2087

Author: neminatha

apps/desktop/desktop/src/main/java/bisq/desktop/common/utils/PageScrollHandler.java

@@ -0,0 +1,106 @@
+package bisq.desktop.common.utils;
+
+import javafx.event.EventHandler;
+import javafx.geometry.Orientation;
+import javafx.scene.control.Control;
+import javafx.scene.control.ScrollBar;
+import javafx.scene.input.KeyCode;
+import javafx.scene.input.KeyEvent;
+import lombok.extern.slf4j.Slf4j;
+import org.fxmisc.easybind.Subscription;
+
+import java.util.Objects;
+import java.util.Optional;
+
+/**
+ * PageScrollHandler
+ * <p>
+ * Utility that installs a key event handler on a JavaFX Control to perform page-wise
+ * scrolling when the PAGE_UP and PAGE_DOWN keys are pressed. The handler locates the
+ * control's vertical ScrollBar (if present) and adjusts its value by the ScrollBar's
+ * block increment, mimicking a page click on the scrollbar trough. The key event is
+ * consumed only when a scroll action is performed.
+ * <p>
+ * Typical usage:
+ * - Instantiate with a JavaFX Control.
+ * - Call {@link #subscribe()} to enable the handler.
+ * - Call {@link #unsubscribe()} to remove the handler.
+ * <p>
+ * Note: methods that inspect or modify the scene graph (e.g. locating ScrollBars)
+ * must be called on the JavaFX Application Thread.
+ */
+@Slf4j
+public class PageScrollHandler implements Subscription {
+
+    private final Control control;
+    private final EventHandler<KeyEvent> keyEventHandler;
+
+    public PageScrollHandler(Control control) {
+        this.control = Objects.requireNonNull(control);
+        this.keyEventHandler = createKeyEventHandler();
+    }
+
+    public static Optional<ScrollBar> findScrollBar(Control control, Orientation orientation) {
+        return findScrollBar(control, orientation, "VirtualScrollBar")
+                .or(() -> findScrollBar(control, orientation, ".scroll-bar"));
+    }
+
+    public static Optional<ScrollBar> findScrollBar(Control control, Orientation orientation, String selector) {
+        if (control.getSkin() == null) {
+            log.warn("Control has no skin; cannot find ScrollBar");
+            return Optional.empty();
+        }
+
+        return control.lookupAll(selector).stream()
+                .filter(node -> node instanceof ScrollBar sb && sb.getOrientation() == orientation)
+                .map(ScrollBar.class::cast)
+                .findFirst();
+    }
+
+    private EventHandler<KeyEvent> createKeyEventHandler() {
+        return event -> {
+            if (event.getCode() == KeyCode.PAGE_UP && blockDecrement()) {
+                event.consume();
+            } else if (event.getCode() == KeyCode.PAGE_DOWN && blockIncrement()) {
+                event.consume();
+            }
+        };
+    }
+
+    public void subscribe() {
+        unsubscribe();
+        control.addEventFilter(KeyEvent.KEY_PRESSED, keyEventHandler);
+    }
+
+    public void unsubscribe() {
+        control.removeEventFilter(KeyEvent.KEY_PRESSED, keyEventHandler);
+    }
+
+    protected boolean blockDecrement() {
+        return findScrollBar(control, Orientation.VERTICAL)
+                .map(this::adjustScrollBarDecrement)
+                .orElse(false);
+    }
+
+    protected boolean blockIncrement() {
+        return findScrollBar(control, Orientation.VERTICAL)
+                .map(this::adjustScrollBarIncrement)
+                .orElse(false);
+    }
+
+    private boolean adjustScrollBarDecrement(ScrollBar vbar) {
+        return adjustScrollBar(vbar, -vbar.getBlockIncrement());
+    }
+
+    private boolean adjustScrollBarIncrement(ScrollBar vbar) {
+        return adjustScrollBar(vbar, vbar.getBlockIncrement());
+    }
+
+    private boolean adjustScrollBar(ScrollBar vbar, double increment) {
+        double oldValue = vbar.getValue();
+        vbar.adjustValue(oldValue + increment);
+        double newValue = vbar.getValue();
+        log.debug("ScrollBar adjusted: {}; {} -> {}", vbar, oldValue, newValue);
+        return oldValue != newValue;
+    }
+}

apps/desktop/desktop/src/main/java/bisq/desktop/main/content/chat/message_container/list/ChatMessagesListView.java

@@ -20,6 +20,7 @@
 import bisq.chat.ChatChannel;
 import bisq.chat.ChatMessage;
 import bisq.desktop.common.ManagedDuration;
+import bisq.desktop.common.utils.PageScrollHandler;
 import bisq.desktop.components.controls.Badge;
 import bisq.desktop.components.controls.BisqTooltip;
 import bisq.desktop.components.list_view.ListViewUtil;
@@ -74,6 +75,7 @@ protected void layoutChildren() {
     private final Label placeholderTitle = new Label("");
     private final Label placeholderDescription = new Label("");
     private final Pane scrollDownBackground;
+    private final PageScrollHandler pageScrollHandler;
     private Optional<ScrollBar> scrollBar = Optional.empty();
     private Subscription hasUnreadMessagesPin, showScrolledDownButtonPin;
     private Timeline fadeInScrollDownBadgeTimeline;
@@ -93,6 +95,8 @@ public ChatMessagesListView(ChatMessagesListModel model, ChatMessagesListControl
         listView.setSelectionModel(new NoSelectionModel<>());
         VBox.setVgrow(listView, Priority.ALWAYS);
 
+        pageScrollHandler = new PageScrollHandler(listView);
+
         scrollDownBackground = new Pane();
         scrollDownBackground.getStyleClass().add("scroll-down-bg");
 
@@ -171,6 +175,7 @@ protected void onViewAttached() {
         model.getLayoutChildrenDone().bind(root.getLayoutChildrenDone());
 
         scrollDownBadge.setOnMouseClicked(e -> controller.onScrollToBottom());
+        pageScrollHandler.subscribe();
     }
 
     @Override
@@ -185,6 +190,7 @@ protected void onViewDetached() {
         model.getLayoutChildrenDone().unbind();
         hasUnreadMessagesPin.unsubscribe();
         showScrolledDownButtonPin.unsubscribe();
+        pageScrollHandler.unsubscribe();
 
         Tooltip.uninstall(scrollDownBadge, scrollDownTooltip);