Override the OSX scrollbar behavior for the image panel.

Forces the use of the "always visible" style of the scrollbars on the
image panel, instead of the possible overlay style, which would hide
portions of the image.

Introduces a platform specific SwtUtil class inside the the SWT package,
so it can easily access protected fields, which can be (sparingly) used
to customize platform specific behavior.

Fixes #1141

gapic/src/main/com/google/gapid/widgets/ImagePanel.java

@@ -25,6 +25,7 @@
 import static com.google.gapid.widgets.Widgets.createToggleToolItem;
 import static com.google.gapid.widgets.Widgets.createToolItem;
 import static com.google.gapid.widgets.Widgets.withSpans;
+import static org.eclipse.swt.widgets.SwtUtil.disableAutoHideScrollbars;
 
 import com.google.common.collect.Lists;
 import com.google.common.collect.Maps;
@@ -440,6 +441,7 @@ public ImageComponent(Composite parent, Theme theme, Consumer<AlphaWarning> show
         boolean naturallyFlipped) {
       super(parent, SWT.BORDER | SWT.V_SCROLL | SWT.H_SCROLL | SWT.NO_BACKGROUND);
       setLayout(new FillLayout(SWT.VERTICAL));
+      disableAutoHideScrollbars(this);
 
       this.showAlphaWarning = showAlphaWarning;
       this.naturallyFlipped = naturallyFlipped;

gapic/src/platform/linux/org/eclipse/swt/widgets/SwtUtil.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright (C) 2017 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.eclipse.swt.widgets;
+
+public class SwtUtil {
+  private SwtUtil() {
+  }
+
+  public static void disableAutoHideScrollbars(@SuppressWarnings("unused") Scrollable widget) {
+    // Do nothing.
+  }
+}

gapic/src/platform/osx/org/eclipse/swt/widgets/SwtUtil.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright (C) 2017 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.eclipse.swt.widgets;
+
+import org.eclipse.swt.internal.cocoa.OS;
+
+public class SwtUtil {
+  private SwtUtil() {
+  }
+
+  public static void disableAutoHideScrollbars(Scrollable widget) {
+    if (OS.VERSION >= 0x1070) {
+      OS.objc_msgSend(widget.scrollView.id,
+          OS.sel_registerName("setScrollerStyle:"), 0 /* NSScrollerStyleLegacy */);
+      widget.scrollView.setAutohidesScrollers(false);
+    }
+  }
+}

gapic/src/platform/windows/org/eclipse/swt/widgets/SwtUtil.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright (C) 2017 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.eclipse.swt.widgets;
+
+public class SwtUtil {
+  private SwtUtil() {
+  }
+
+  public static void disableAutoHideScrollbars(@SuppressWarnings("unused") Scrollable widget) {
+    // Do nothing.
+  }
+}