update(panel): constrain panel to viewport boundries

Prevents the panel from going outside the viewport by adjusting the position.
If developers want more control over how the panel gets repositioned, they can specify addition fallback positions via `addPanelPosition`.

Related to angular#9641.

Fixes angular#7878.

Author: crisbeto

src/components/panel/panel.js

@@ -2156,6 +2156,12 @@ MdPanelPosition.absPosition = {
   LEFT: 'left'
 };
 
+/**
+ * Margin between the edges of a panel and the viewport.
+ * @const {number}
+ */
+MdPanelPosition.viewportMargin = 8;
+
 
 /**
  * Sets absolute positioning for the panel.
@@ -2525,6 +2531,9 @@ MdPanelPosition.prototype._reduceTranslateValues =
  * @private
  */
 MdPanelPosition.prototype._setPanelPosition = function(panelEl) {
+  // Remove the class in case it has been added before.
+  panelEl.removeClass('_md-panel-position-adjusted');
+
   // Only calculate the position if necessary.
   if (this._absolute) {
     return;
@@ -2539,12 +2548,49 @@ MdPanelPosition.prototype._setPanelPosition = function(panelEl) {
     this._actualPosition = this._positions[i];
     this._calculatePanelPosition(panelEl, this._actualPosition);
     if (this._isOnscreen(panelEl)) {
-      break;
+      return;
     }
   }
+
+  // Class that can be used to re-style the panel if it was repositioned.
+  panelEl.addClass('_md-panel-position-adjusted');
+  this._constrainToViewport(panelEl);
 };
 
 
+/**
+ * Constrains a panel's position to the viewport.
+ * @param {!angular.JQLite} panelEl
+ * @private
+ */
+MdPanelPosition.prototype._constrainToViewport = function(panelEl) {
+  var margin = MdPanelPosition.viewportMargin;
+
+  if (this.getTop()) {
+    var top = parseInt(this.getTop());
+    var bottom = panelEl[0].offsetHeight + top;
+    var viewportHeight = this._$window.innerHeight;
+
+    if (top < margin) {
+      this._top = margin + 'px';
+    } else if (bottom > viewportHeight) {
+      this._top = top - (bottom - viewportHeight + margin) + 'px';
+    }
+  }
+
+  if (this.getLeft()) {
+    var left = parseInt(this.getLeft());
+    var right = panelEl[0].offsetWidth + left;
+    var viewportWidth = this._$window.innerWidth;
+
+    if (left < margin) {
+      this._left = margin + 'px';
+    } else if (right > viewportWidth) {
+      this._left = left - (right - viewportWidth + margin) + 'px';
+    }
+  }
+};
+
 /**
  * Switches between 'start' and 'end'.
  * @param {string} position Horizontal position of the panel