Adds support for -1 row index Escalator spacers (#16644)

Change-Id: Iced2f089785983ce2ef1d2225517156cec4a7db4

2 files changed, 11 insertions, 3 deletions

diff --git a/client/src/com/vaadin/client/widget/escalator/RowContainer.java b/client/src/com/vaadin/client/widget/escalator/RowContainer.java
index e2baf9a03b..b7aae1f4f3 100644
--- a/client/src/com/vaadin/client/widget/escalator/RowContainer.java
+++ b/client/src/com/vaadin/client/widget/escalator/RowContainer.java
@@ -57,7 +57,8 @@ public interface RowContainer {
          * 
          * @param rowIndex
          *            the row index for the spacer to modify. The affected
-         *            spacer is underneath the given index
+         *            spacer is underneath the given index. Use -1 to insert a
+         *            spacer before the first row
          * @param height
          *            the pixel height of the spacer. If {@code height} is
          *            negative, the affected spacer (if exists) will be removed
diff --git a/client/src/com/vaadin/client/widgets/Escalator.java b/client/src/com/vaadin/client/widgets/Escalator.java
index 1d7d9a9910..9b11e7aec1 100644
--- a/client/src/com/vaadin/client/widgets/Escalator.java
+++ b/client/src/com/vaadin/client/widgets/Escalator.java
@@ -4453,8 +4453,15 @@ public class Escalator extends Widget implements RequiresResize,
                             .getScrollSize() + heightDiff);
                 }
 
+                /*
+                 * Don't modify the scrollbars if we're expanding the -1 spacer
+                 * while we're scrolled to the top.
+                 */
+                boolean minusOneSpacerException = spacerIsGrowing
+                        && getRow() == -1 && body.getTopRowLogicalIndex() == 0;
+
                 boolean viewportNeedsScrolling = getRow() < body
-                        .getTopRowLogicalIndex();
+                        .getTopRowLogicalIndex() && !minusOneSpacerException;
                 if (viewportNeedsScrolling) {
 
                     /*
@@ -4570,7 +4577,7 @@ public class Escalator extends Widget implements RequiresResize,
         public void setSpacer(int rowIndex, double height)
                 throws IllegalArgumentException {
 
-            if (rowIndex < 0 || rowIndex >= getBody().getRowCount()) {
+            if (rowIndex < -1 || rowIndex >= getBody().getRowCount()) {
                 throw new IllegalArgumentException("invalid row index: "
                         + rowIndex + ", while the body only has "
                         + getBody().getRowCount() + " rows.");