Fix drop indicator when Grid is scrolled (#9417)

Now the DnD events are listened from tablewrapper element,
which contains also grid's header and footer, making it possible to
drop on top of them.

5 files changed, 72 insertions, 28 deletions

diff --git a/client/src/main/java/com/vaadin/client/connectors/grid/GridDropTargetConnector.java b/client/src/main/java/com/vaadin/client/connectors/grid/GridDropTargetConnector.java
index dbaaefde91..33729afe77 100644
--- a/client/src/main/java/com/vaadin/client/connectors/grid/GridDropTargetConnector.java
+++ b/client/src/main/java/com/vaadin/client/connectors/grid/GridDropTargetConnector.java
@@ -22,7 +22,6 @@ import java.util.Objects;
 import com.google.gwt.dom.client.Element;
 import com.google.gwt.dom.client.NativeEvent;
 import com.google.gwt.dom.client.TableRowElement;
-import com.google.gwt.dom.client.TableSectionElement;
 import com.google.gwt.user.client.Window;
 import com.vaadin.client.ServerConnector;
 import com.vaadin.client.WidgetUtil;
@@ -82,10 +81,11 @@ public class GridDropTargetConnector extends DropTargetExtensionConnector {
     private String styleDragEmpty;
 
     /**
-     * The latest row that was dragged on top of, or the grid body if drop is
-     * not applicable for any rows. Need to store this so that can remove drop
-     * hint styling when the target has changed since all browsers don't seem to
-     * always fire the drag-enter drag-exit events in a consistent order.
+     * The latest row that was dragged on top of, or the tablewrapper element
+     * returned by {@link #getDropTargetElement()} if drop is not applicable for
+     * any body rows. Need to store this so that can remove drop hint styling
+     * when the target has changed since all browsers don't seem to always fire
+     * the drag-enter drag-exit events in a consistent order.
      */
     private Element latestTargetElement;
 
@@ -106,7 +106,7 @@ public class GridDropTargetConnector extends DropTargetExtensionConnector {
 
         Element targetElement = getTargetElement(
                 (Element) dropEvent.getEventTarget().cast());
-        // the target element is either the body or one of the rows
+        // the target element is either the tablewrapper or one of the body rows
         if (TableRowElement.is(targetElement)) {
             rowKey = getRowData(targetElement.cast())
                     .getString(GridState.JSONKEY_ROWKEY);
@@ -237,20 +237,43 @@ public class GridDropTargetConnector extends DropTargetExtensionConnector {
     }
 
     private Element getTargetElement(Element source) {
+        final Element tableWrapper = getDropTargetElement();
         final BodyRowContainer gridBody = getGridBody();
-        final TableSectionElement bodyElement = gridBody.getElement();
-        while (!Objects.equals(source, bodyElement)) {
+        final int rowCount = gridBody.getRowCount();
+
+        while (!Objects.equals(source, tableWrapper)) {
+            // the drop might happen on top of header, body or footer rows
             if (TableRowElement.is(source)) {
-                return source;
+                String parentTagName = source.getParentElement().getTagName();
+                if ("thead".equalsIgnoreCase(parentTagName)) {
+                    // for empty grid or ON_TOP mode, drop as last row,
+                    // otherwise as above first visible row
+                    if (rowCount == 0
+                            || getState().dropMode == DropMode.ON_TOP) {
+                        return tableWrapper;
+                    } else {
+                        return gridBody.getRowElement(0);
+                    }
+                } else if ("tfoot".equalsIgnoreCase(parentTagName)) {
+                    // for empty grid or ON_TOP mode, drop as last row,
+                    // otherwise as below last visible row
+                    if (rowCount == 0
+                            || getState().dropMode == DropMode.ON_TOP) {
+                        return tableWrapper;
+                    } else {
+                        return gridBody.getRowElement(rowCount - 1);
+                    }
+                } else { // parent is tbody
+                    return source;
+                }
             }
             source = source.getParentElement();
         }
-        // the drag is on top of the body
-        final int rowCount = gridBody.getRowCount();
-        // if no rows in grid, or if the drop mode is on top, then there is no
+        // the drag is on top of the tablewrapper
+        // if no rows in grid, or if the drop mode is ON_TOP, then there is no
         // target row for the drop
         if (rowCount == 0 || getState().dropMode == DropMode.ON_TOP) {
-            return bodyElement;
+            return tableWrapper;
         } else { // if dragged under the last row to empty space, drop target
                  // needs to be below the last row
             return gridBody.getRowElement(rowCount - 1);
@@ -259,7 +282,14 @@ public class GridDropTargetConnector extends DropTargetExtensionConnector {
 
     @Override
     protected Element getDropTargetElement() {
-        return getGridBody().getElement();
+        /*
+         * The drop target element, the <div class="v-grid-tablewrapper" />.
+         * This is where the event listeners are added since then we can accept
+         * drops on header, body and footer rows and the "empty area" outside
+         * rows. Also it is used since then the drop hints for "empty" area can
+         * be shown properly as the grid body would scroll.
+         */
+        return getEscalator().getTableWrapper();
     }
 
     private Escalator getEscalator() {
diff --git a/client/src/main/java/com/vaadin/client/widgets/Escalator.java b/client/src/main/java/com/vaadin/client/widgets/Escalator.java
index 5239fa9636..1a96f7b771 100644
--- a/client/src/main/java/com/vaadin/client/widgets/Escalator.java
+++ b/client/src/main/java/com/vaadin/client/widgets/Escalator.java
@@ -6796,6 +6796,20 @@ public class Escalator extends Widget
         return null;
     }
 
+    /**
+     * Returns the {@code <div class="v-grid-tablewrapper" />} element which has
+     * the table inside it.
+     * <p>
+     * <em>NOTE: you should not do any modifications to the returned element.
+     * This API is only available for querying data from the element.</em>
+     *
+     * @return the table wrapper element
+     * @since 8.1
+     */
+    public Element getTableWrapper() {
+        return tableWrapper;
+    }
+
     private Element getSubPartElementTableStructure(SubPartArguments args) {
 
         String type = args.getType();
diff --git a/documentation/advanced/advanced-dragndrop.asciidoc b/documentation/advanced/advanced-dragndrop.asciidoc
index 10d4ce2b01..8aa18b72ac 100644
--- a/documentation/advanced/advanced-dragndrop.asciidoc
+++ b/documentation/advanced/advanced-dragndrop.asciidoc
@@ -348,14 +348,16 @@ dropTarget.addGridDropListener(event -> {
 
 The _drop location_ property in the [classname]#GridDropEvent# specifies the dropped location in relative to grid row the drop happened on and depends on the used [classname]#DropMode#. When the drop happened on top of a row, the possible options for the location are `ON_TOP`, `ABOVE` and `BELOW`.
 
-If the grid is empty, or if there was empty space after the last row in grid and the [classname]#DropMode.ON_TOP# was used, then the drop location `EMPTY` will be used. If the drop modes [classname]#DropMode.BETWEEN# or [classname]#DropMode.ON_TOP_OR_BETWEEN# are used, then the location can be `EMPTY` only when the grid was empty; otherwise the drop happened ´BELOW´ the last row. When the drop location is `EMPTY`, the [methodname]#getDropTargetRow# method will also return an empty optional.
+If the grid is empty or if the drop was on empty space after the last row in grid, and the [classname]#DropMode.ON_TOP# was used, then the drop location `EMPTY` will be used. If the drop modes [classname]#DropMode.BETWEEN# or [classname]#DropMode.ON_TOP_OR_BETWEEN# are used, then the location can be `EMPTY` only when the grid was empty; otherwise the drop happened `BELOW` the last visible row. When the drop location is `EMPTY`, the [methodname]#getDropTargetRow# method will also return an empty optional.
+
+When dropping on top of the grid's header or footer, the drop location will be `EMPTY` if there are no rows in the grid or if [classname]#DropMode.ON_TOP# was used. If there are rows in the grid, dropping on top of the header will set the drop location to `ABOVE` and the dropped row will be the first currently visible row in grid. Similarly, if dropping on top of the footer, the drop location will be `BELOW` and the dropped row will be the last visible row in the grid.
 
 ==== CSS Style Rules
 
 A drop target Grid's body has the style name `v-grid-body-droptarget` to indicate that it is a potential target for data to be dropped.
 
 When dragging data over a drop target Grid's row, depending on the drop mode and the mouse position relative to the row, a style name is applied to the row or to the grid body to indicate the drop location.
-When dragging on top of a row, `v-grid-row-drag-center` indicates ON_TOP, `v-grid-row-drag-top` indicates ABOVE and `v-grid-row-drag-bottom` indicates BELOW locations. When dragging on top of an empty grid, or when the drop location is ON_TOP and dragged below the last row in grid (and there is empty space visible), the `v-grid-body-body-drag-top` style is applied to the table body element.
+When dragging on top of a row, `v-grid-row-drag-center` indicates ON_TOP, `v-grid-row-drag-top` indicates ABOVE and `v-grid-row-drag-bottom` indicates BELOW locations. When dragging on top of an empty grid, or when the drop location is ON_TOP and dragged below the last row in grid (and there is empty space visible), the `v-grid-body-body-drag-top` style is applied to the `v-grid-tablewrapper` element which surrounds the grid header, body and footer.
 
 (((range="endofrange", startref="term.advanced.dragndrop")))
 
diff --git a/server/src/main/java/com/vaadin/ui/components/grid/GridDropEvent.java b/server/src/main/java/com/vaadin/ui/components/grid/GridDropEvent.java
index e6af56d66d..aa80cdc7b8 100644
--- a/server/src/main/java/com/vaadin/ui/components/grid/GridDropEvent.java
+++ b/server/src/main/java/com/vaadin/ui/components/grid/GridDropEvent.java
@@ -87,8 +87,9 @@ public class GridDropEvent<T> extends DropEvent<Grid<T>> {
      * Get the location of the drop within the row.
      * <p>
      * <em>NOTE: when dropped on an empty grid, or when {@link DropMode#ON_TOP}
-     * is used and the drop happened on empty space after last row, the location
-     * will be {@link DropLocation#EMPTY}.</em>
+     * is used and the drop happened on empty space after last row or on top of
+     * the header / footer, the location will be
+     * {@link DropLocation#EMPTY}.</em>
      *
      * @return Location of the drop within the row.
      * @see GridDropTarget#setDropMode(DropMode)
diff --git a/themes/src/main/themes/VAADIN/themes/valo/components/_grid.scss b/themes/src/main/themes/VAADIN/themes/valo/components/_grid.scss
index 2f6be4a584..fdc4eff73f 100644
--- a/themes/src/main/themes/VAADIN/themes/valo/components/_grid.scss
+++ b/themes/src/main/themes/VAADIN/themes/valo/components/_grid.scss
@@ -832,7 +832,7 @@ $v-grid-drag-indicator-color: $v-focus-color;
 
   // Drag and drop
   .#{$primary-stylename}-row-drag-top, .#{$primary-stylename}-row-drag-bottom, .#{$primary-stylename}-row-drag-bottom {
-    z-index: 100;
+    z-index: 100; // based on what else z-indexes are in grid/escalator
   }
 
   .#{$primary-stylename}-row-drag-top:before,
@@ -855,6 +855,10 @@ $v-grid-drag-indicator-color: $v-focus-color;
     top: -1px;
   }
 
+  .#{$primary-stylename}-row-drag-top:first-child:before {
+    top: 0;
+  }
+
   .#{$primary-stylename}-row-drag-center:after {
     content: "";
     position: absolute;
@@ -877,28 +881,21 @@ $v-grid-drag-indicator-color: $v-focus-color;
     }
   }
 
-  // Expand Grid's body to cover the whole Grid
-  .#{$primary-stylename}-body-droptarget {
-    top: 0;
-    right: 0;
-    bottom: 0;
-    left: 0;
-  }
   // Show drop hint when the grid is empty or,
   // if the DropMode.ON_TOP is used and dragging below last row
   .#{$primary-stylename}-body-drag-top:after {
     content: "";
     position: absolute;
     top: 0;
-    right: 2px;
+    right: 0;
     bottom: 0;
     left: 0;
     pointer-events: none;
     border: 2px solid $v-grid-drag-indicator-color;
+    z-index: 100; // based on what else z-indexes are in grid/escalator
   }
 }
 
-
 @include keyframes(valo-grid-editor-footer-animate-in) {
   0% {
     margin-top: -$v-grid-row-height;