Fix Button as a Html5 DragSource (#9346)

* Fix Button as a Html5 DragSourcei

It was completely broken, not being clickable, and also preventing anything else from being clicked.

client/src/main/java/com/vaadin/client/ui/VButton.java

                 clickPending = true;
                 setFocus(true);
                 DOM.setCapture(getElement());
-                isCapturing = isCapturingEnabled();
+                isCapturing = true;
                 addStyleName(CLASSNAME_PRESSED);
             }
             break;
             break;
         case Event.ONMOUSEMOVE:
             clickPending = false;
-            if (isCapturing) {
+            if (isCapturing && !isDraggable()) {
                 // Prevent dragging (on other browsers);
                 DOM.eventPreventDefault(event);
             }
     }
 
     /**
-     * Enables or disables the widget's capturing of mouse events with the mouse
-     * held down.
-     *
-     * @param enabled
-     *            {@literal true} if capturing enabled, {@literal false}
-     *            otherwise
+     * Returns if this button has been made <code>draggable</code> or not.
      *
-     * @since 8.1
-     */
-    public void setCapturingEnabled(boolean enabled) {
-        capturingEnabled = enabled;
-    }
-
-    /**
-     * Returns if the widget's capturing of mouse events are enabled.
-     *
-     * @return {@literal true} if mouse capturing is enabled, {@literal false}
+     * @return {@literal true} if draggable is enabled, {@literal false}
      *         otherwise
-     *
-     * @since 8.1
      */
-    public boolean isCapturingEnabled() {
-        return capturingEnabled;
+    private boolean isDraggable() {
+        return getElement().getPropertyBoolean("draggable");
     }
 
     private static native int getHorizontalBorderAndPaddingWidth(Element elem)

client/src/main/java/com/vaadin/client/ui/button/ButtonConnector.java

 
     }
 
-    @Override
-    public void onDragSourceAttached() {
-        // Disable mouse event capturing to make the widget draggable
-        getWidget().setCapturingEnabled(false);
-    }
-
-    @Override
-    public void onDragSourceDetached() {
-        // Reset mouse event capturing
-        getWidget().setCapturingEnabled(true);
-    }
 }

uitest/src/main/java/com/vaadin/tests/dnd/DraggableButton.java

  */
 package com.vaadin.tests.dnd;
 
+import com.vaadin.annotations.Widgetset;
 import com.vaadin.server.Page;
 import com.vaadin.server.VaadinRequest;
 import com.vaadin.tests.components.AbstractTestUIWithLog;
 import com.vaadin.ui.dnd.DragSourceExtension;
 import com.vaadin.ui.dnd.DropTargetExtension;
 
+@Widgetset("com.vaadin.DefaultWidgetSet")
 public class DraggableButton extends AbstractTestUIWithLog {
 
     @Override
     protected void setup(VaadinRequest request) {
 
-        Button draggableButton = new Button("Draggable Button");
+        Button draggableButton = new Button("Draggable Button",
+                event -> log("clicked draggable button"));
         DragSourceExtension<Button> dragSourceExtension = new DragSourceExtension<>(
                 draggableButton);
         dragSourceExtension.setDataTransferText(
                 .addDropListener(event -> log(event.getDataTransferText()));
 
         Layout layout = new HorizontalLayout();
-        layout.addComponents(draggableButton, dropTarget);
+        layout.addComponents(draggableButton, dropTarget, new Button(
+                "another button", event -> log("click on another button")));
         addComponent(layout);
 
         // Add styling
 
     @Override
     protected String getTestDescription() {
-        return "Test if Button is draggable";
+        return "Test if Button is draggable, and it won't steal all other clicks";
     }
 }