Don't block mouse click events on the image.

Doing so means that when the focus is on one of the text boxes,
clicking on the image doesn't remove the focus from the text box,
which is annoying as change of focus is often used to trigger an
update of the graph.

Also increase the minimum selection size to avoid unintentional
drag-to-zoom events, which happen easily with a touch pad.

src/tsd/client/QueryUi.java

@@ -1084,7 +1084,6 @@ private final class ZoomBox extends HTML
 
     @Override
     public void onMouseDown(final MouseDownEvent event) {
-      event.preventDefault();
       // Check if the zoom selection is active, if so, it's possible that the
       // mouse left the browser mid-selection and got stuck enabled even
       // though the mouse isn't still pressed. If that's the case, do a similar
@@ -1114,10 +1113,6 @@ public void onMouseDown(final MouseDownEvent event) {
     public void onMouseMove(final MouseMoveEvent event) {
       event.preventDefault();
 
-      if (!zoom_selection_active) {
-        return;
-      }
-
       final int x = event.getRelativeX(graph.getElement());
       final int y = event.getRelativeY(graph.getElement());
       int left;
@@ -1152,8 +1147,6 @@ public void onMouseMove(final MouseMoveEvent event) {
 
     @Override
     public void onMouseUp(final MouseUpEvent event) {
-      event.preventDefault();
-
       if (zoom_selection_active) {
         endSelection(event);
       }
@@ -1206,7 +1199,7 @@ private <H extends EventHandler> void endSelection(final MouseEvent<H> event) {
       // Prevent division by zero if image is pathologically small.
       // or: Prevent changing anything if the distance the cursor traveled was
       // too small (as happens during a simple click or unintentional click).
-      if (actual_width < 1 || end_x - start_x < 2) {
+      if (actual_width < 1 || end_x - start_x <= 5) {
         return;
       }