diff --git a/browser/components/wintaskbar/WindowsPreviewPerTab.jsm b/browser/components/wintaskbar/WindowsPreviewPerTab.jsm
index b66f597e1db..1f264c1ce9a 100644
--- a/browser/components/wintaskbar/WindowsPreviewPerTab.jsm
+++ b/browser/components/wintaskbar/WindowsPreviewPerTab.jsm
@@ -132,6 +132,19 @@ function getFaviconAsImage(iconurl, callback) {
     _imageFromURI(faviconSvc.defaultFavicon, callback);
 }
 
+// Snaps the given rectangle to be pixel-aligned at the given scale
+function snapRectAtScale(r, scale) {
+  let x = Math.floor(r.x * scale);
+  let y = Math.floor(r.y * scale);
+  let width = Math.ceil((r.x + r.width) * scale) - x;
+  let height = Math.ceil((r.y + r.height) * scale) - y;
+
+  r.x = x / scale;
+  r.y = y / scale;
+  r.width = width / scale;
+  r.height = height / scale;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 //// PreviewController
 
@@ -172,6 +185,13 @@ function PreviewController(win, tab) {
       dirtyRegion.init();
       return dirtyRegion;
     });
+
+  XPCOMUtils.defineLazyGetter(this, "winutils",
+    function () {
+      let win = tab.linkedBrowser.contentWindow;
+      return win.QueryInterface(Ci.nsIInterfaceRequestor)
+                .getInterface(Ci.nsIDOMWindowUtils);
+  });
 }
 
 PreviewController.prototype = {
@@ -214,6 +234,12 @@ PreviewController.prototype = {
     this.canvasPreview.height = 0;
   },
 
+  get zoom() {
+    // We use this property instead of the fullZoom property because this
+    // accurately reflects the actual zoom factor used when drawing.
+    return this.winutils.screenPixelsPerCSSPixel;
+  },
+
   // Updates the controller's canvas with the parts of the <browser> that need
   // to be redrawn.
   updateCanvasPreview: function () {
@@ -223,23 +249,30 @@ PreviewController.prototype = {
     if (bx.width != this.canvasPreview.width ||
         bx.height != this.canvasPreview.height) {
       // Invalidate the entire area and repaint
-      this.onTabPaint({left:0, top:0, width:bx.width, height:bx.height});
+      this.onTabPaint({left:0, top:0, right:win.innerWidth, bottom:win.innerHeight});
       this.canvasPreview.width = bx.width;
       this.canvasPreview.height = bx.height;
     }
 
     // Draw dirty regions
     let ctx = this.canvasPreview.getContext("2d");
+    let scale = this.zoom;
+
     let flags = this.canvasPreviewFlags;
-    // width/height are occasionally bogus and too large for drawWindow
-    // so we clip to the canvas region
-    this.dirtyRegion.intersectRect(0, 0, bx.width, bx.height);
+    // The dirty region may include parts that are offscreen so we clip to the
+    // canvas area.
+    this.dirtyRegion.intersectRect(0, 0, win.innerWidth, win.innerHeight);
     this.dirtyRects.forEach(function (r) {
+      // We need to snap the rectangle to be pixel aligned in the destination
+      // coordinate space. Otherwise natively themed widgets might not draw.
+      snapRectAtScale(r, scale);
       let x = r.x;
       let y = r.y;
       let width = r.width;
       let height = r.height;
+
       ctx.save();
+      ctx.scale(scale, scale);
       ctx.translate(x, y);
       ctx.drawWindow(win, x, y, width, height, "white", flags);
       ctx.restore();
@@ -252,16 +285,11 @@ PreviewController.prototype = {
   },
 
   onTabPaint: function (rect) {
-    // Ignore spurious dirty rects
-    if (!rect.width || !rect.height)
-      return;
-
-    let r = { x: Math.floor(rect.left),
-              y: Math.floor(rect.top),
-              width: Math.ceil(rect.width),
-              height: Math.ceil(rect.height)
-            };
-    this.dirtyRegion.unionRect(r.x, r.y, r.width, r.height);
+    let x = Math.floor(rect.left),
+        y = Math.floor(rect.top),
+        width = Math.ceil(rect.right) - x,
+        height = Math.ceil(rect.bottom) - y;
+    this.dirtyRegion.unionRect(x, y, width, height);
   },
 
   updateTitleAndTooltip: function () {
diff --git a/gfx/thebes/gfxContext.cpp b/gfx/thebes/gfxContext.cpp
index eb4d6cddee7..5c488f44319 100644
--- a/gfx/thebes/gfxContext.cpp
+++ b/gfx/thebes/gfxContext.cpp
@@ -405,11 +405,15 @@ gfxContext::UserToDevicePixelSnapped(gfxRect& rect, PRBool ignoreScale) const
     // if we're not at 1.0 scale, don't snap, unless we're
     // ignoring the scale.  If we're not -just- a scale,
     // never snap.
+    const gfxFloat epsilon = 0.0000001;
+#define WITHIN_E(a,b) (fabs((a)-(b)) < epsilon)
     cairo_matrix_t mat;
     cairo_get_matrix(mCairo, &mat);
     if (!ignoreScale &&
-        (mat.xx != 1.0 || mat.yy != 1.0 || mat.xy != 0.0 || mat.yx != 0.0))
+        (!WITHIN_E(mat.xx,1.0) || !WITHIN_E(mat.yy,1.0) ||
+         !WITHIN_E(mat.xy,0.0) || !WITHIN_E(mat.yx,0.0)))
         return PR_FALSE;
+#undef WITHIN_E
 
     gfxPoint p1 = UserToDevice(rect.pos);
     gfxPoint p2 = UserToDevice(rect.pos + gfxSize(rect.size.width, 0.0));