Fix/workaround for unwanted ScrollPane blurring

app/display/representation-javafx/src/main/java/org/csstudio/display/builder/representation/javafx/widgets/NavigationTabs.java

@@ -26,6 +26,7 @@
 import javafx.scene.layout.VBox;
 import javafx.scene.paint.Color;
 import javafx.scene.text.Font;
+import org.phoebus.ui.javafx.NonCachingScrollPane;
 
 /** Navigation Tabs
  *
@@ -95,7 +96,7 @@ public static interface Listener
     public NavigationTabs()
     {
         // Scroll pane in case body exceeds size of this, the other BorderPane
-        final ScrollPane scroll = new ScrollPane(body);
+        final ScrollPane scroll = new NonCachingScrollPane(body);
         scroll.getStyleClass().add("navtab_scroll");
 
         // Inner border pane to auto-resize 'body' and add border + padding via style sheet

core/ui/src/main/java/org/phoebus/ui/javafx/NonCachingScrollPane.java

@@ -0,0 +1,84 @@
+/*
+ * Copyright (C) 2023 European Spallation Source ERIC.
+ *
+ *  This program is free software; you can redistribute it and/or
+ *  modify it under the terms of the GNU General Public License
+ *  as published by the Free Software Foundation; either version 2
+ *  of the License, or (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program; if not, write to the Free Software
+ *  Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
+ *
+ */
+
+package org.phoebus.ui.javafx;
+
+import javafx.scene.Node;
+import javafx.scene.control.ScrollPane;
+import javafx.scene.control.Skin;
+import javafx.scene.control.skin.ScrollPaneSkin;
+import javafx.scene.layout.StackPane;
+
+import java.lang.reflect.Field;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+
+/**
+ * A {@link ScrollPane} subclass disabling caching on the (probably) viewport. Motivation is that
+ * under certain circumstances the contents of a {@link ScrollPane} gets blurred when zooming.
+ * See <a href="https://bugs.openjdk.org/browse/JDK-8089499">this OpenJDK bug</a> and this
+ * <a href="https://github.com/ControlSystemStudio/phoebus/issues/2864">Github issue</a>.
+ * The implementation - based on the OpenJDK bug ticket - depends on reflection to access a private field
+ * in {@link ScrollPaneSkin}. As per November 2023 the issue is not fixed in JavaFX version 21.
+ * <p>
+ * In case the addressed field is not available or accessible (e.g. due JavaFX updates), exceptions are logged,
+ * but not propagated, in which case this subclass inherits the behavior of {@link ScrollPane}.
+ * </p>
+ */
+public class NonCachingScrollPane extends ScrollPane {
+
+    public NonCachingScrollPane(Node content) {
+        super(content);
+    }
+
+    @Override
+    protected Skin<?> createDefaultSkin() {
+        try {
+            return new NonCachingScrollPaneSkin(this);
+        } catch (Exception e) {
+            // Java reflection magic failed, fall back to default skin.
+            return super.createDefaultSkin();
+        }
+    }
+
+    /**
+     * {@link ScrollPaneSkin} subclass that attempts to retrieve the private field <code>viewRect</code>
+     * of the {@link ScrollPaneSkin} class, and then set the caching of that skin to <code>false</code>.
+     * If the field cannot be accessed, a {@link RuntimeException} is thrown in the constructor.
+     */
+    private static class NonCachingScrollPaneSkin extends ScrollPaneSkin {
+
+        public NonCachingScrollPaneSkin(final ScrollPane scrollpane) {
+            super(scrollpane);
+            StackPane viewRect;
+            try {
+                Field viewRectField = ScrollPaneSkin.class.getDeclaredField("viewRect");
+                viewRectField.setAccessible(true);
+                viewRect = (StackPane) viewRectField.get(this);
+                viewRect.setCache(false);
+            }
+            catch (Exception e) {
+                Logger.getLogger(NonCachingScrollPane.class.getName()).log(Level.WARNING,
+                        "Unable to find field viewRect via reflection",
+                        e);
+                throw new RuntimeException(e);
+            }
+        }
+    }
+}