Ensure that the help popups won't run off the screen if opened right near the...

Ensure that the help popups won't run off the screen if opened right near the bottom of the viewport. Fixes bug #1787.

Note if the popup is bigger than the viewport then things get weird. I don't think that case is worth worrying about.

htdocs/js/mahara.js

@@ -354,6 +354,7 @@ function contextualHelp(formName, helpName, pluginType, pluginName, page, sectio
     if (contextualHelpCache[key]) {
         buildContextualHelpBox(contextualHelpCache[key]);
         callLater(0, function() { contextualHelpOpened = true; });
+        ensureHelpIsOnScreen(contextualHelpContainer, position);
     }
     else {
         if (contextualHelpDeferrable && contextualHelpDeferrable.cancel) {
@@ -371,6 +372,7 @@ function contextualHelp(formName, helpName, pluginType, pluginName, page, sectio
                 buildContextualHelpBox(contextualHelpCache[key]);
             }
             contextualHelpOpened = true;
+            ensureHelpIsOnScreen(contextualHelpContainer, position);
             processingStop();
         },
         function (error) {
@@ -397,6 +399,19 @@ function buildContextualHelpBox(content) {
     connect('helpstop', 'onclick', function(e) { e.stop(); });
 }
 
+/*
+ * Ensures that the contextual help box given is fully visible on screen. This
+ * will adjust the position of the help vertically if the help has opened right
+ * next to the bottom or top of the viewport
+ */
+function ensureHelpIsOnScreen(container, position) {
+    var dimensions = getElementDimensions(container);
+    if (position.y + dimensions.h > screenDimensions.h + getFirstElementByTagAndClassName('html').scrollTop) {
+        position.y -= dimensions.h - 18;
+        setElementPosition(container, position);
+    }
+}
+
 /* Only works in non-ie at the moment. Using 'document' as the element
    makes IE detect the event, but then makes it so you need to click on
    the help twice before it opens. */