Index: src/wp-admin/css/common.css
===================================================================
--- src/wp-admin/css/common.css	(revision 35372)
+++ src/wp-admin/css/common.css	(working copy)
@@ -2696,21 +2696,40 @@
 body.plugins-php #TB_window,
 body.update-core-php #TB_window,
 body.index-php #TB_window {
-	background: #fcfcfc url( ../images/spinner.gif ) no-repeat center;
+	background: #fcfcfc;
 }
 
+body.plugin-install-php #TB_window.thickbox-loading:before,
+body.import-php #TB_window.thickbox-loading:before,
+body.plugins-php #TB_window.thickbox-loading:before,
+body.update-core-php #TB_window.thickbox-loading:before,
+body.index-php #TB_window.thickbox-loading:before {
+	content: "";
+	display: block;
+	width: 20px;
+	height: 20px;
+	position: absolute;
+	left: 50%;
+	top: 50%;
+	z-index: -1;
+	margin: -10px 0 0 -10px;
+	-webkit-transform: translateZ(0);
+	transform: translateZ(0);
+	background: #fcfcfc url(../images/spinner.gif) no-repeat center;
+	-webkit-background-size: 20px 20px;
+	background-size: 20px 20px;
+}
+
 @media print,
 	(-webkit-min-device-pixel-ratio: 1.25),
 	(min-resolution: 120dpi) {
 
-	body.plugin-install-php #TB_window,
-	body.import-php #TB_window,
-	body.plugins-php #TB_window,
-	body.update-core-php #TB_window,
-	body.index-php #TB_window {
-		background: #fcfcfc url( ../images/spinner-2x.gif ) no-repeat center;
-		-webkit-background-size: 20px 20px;
-		background-size: 20px 20px;
+	body.plugin-install-php #TB_window.thickbox-loading:before,
+	body.import-php #TB_window.thickbox-loading:before,
+	body.plugins-php #TB_window.thickbox-loading:before,
+	body.update-core-php #TB_window.thickbox-loading:before,
+	body.index-php #TB_window.thickbox-loading:before {
+		background: #fcfcfc url(../images/spinner-2x.gif) no-repeat center;
 	}
 }
 
Index: src/wp-includes/js/thickbox/thickbox.css
===================================================================
--- src/wp-includes/js/thickbox/thickbox.css	(revision 35372)
+++ src/wp-includes/js/thickbox/thickbox.css	(working copy)
@@ -12,7 +12,7 @@
 
 #TB_window {
 	position: fixed;
-	background: #fff;
+	background-color: #fff;
 	z-index: 100050; /* Above DFW. */
 	visibility: hidden;
 	text-align: left;
@@ -55,7 +55,7 @@
 	float: left;
 	font-weight: 600;
 	line-height: 29px;
-	overflow: hidden; 
+	overflow: hidden;
 	padding: 0 29px 0 10px;
 	text-overflow: ellipsis;
 	white-space: nowrap;
Index: src/wp-includes/js/thickbox/thickbox.js
===================================================================
--- src/wp-includes/js/thickbox/thickbox.js	(revision 35372)
+++ src/wp-includes/js/thickbox/thickbox.js	(working copy)
@@ -18,9 +18,16 @@
 	imgLoader.src = tb_pathToImage;
 });
 
-//add thickbox to href & area elements that have a class of .thickbox
+/*
+ * Add thickbox to href & area elements that have a class of .thickbox.
+ * Remove the loading indicator when content in an iframe has loaded.
+ */
 function tb_init(domChunk){
-	jQuery('body').on('click', domChunk, tb_click);
+	jQuery( 'body' )
+		.on( 'click', domChunk, tb_click )
+		.on( 'thickbox:iframe:loaded', function() {
+			jQuery( '#TB_window' ).removeClass( 'thickbox-loading' );
+		});
 }
 
 function tb_click(){
@@ -39,12 +46,12 @@
 			jQuery("body","html").css({height: "100%", width: "100%"});
 			jQuery("html").css("overflow","hidden");
 			if (document.getElementById("TB_HideSelect") === null) {//iframe to hide select elements in ie6
-				jQuery("body").append("<iframe id='TB_HideSelect'>"+thickboxL10n.noiframes+"</iframe><div id='TB_overlay'></div><div id='TB_window'></div>");
+				jQuery("body").append("<iframe id='TB_HideSelect'>"+thickboxL10n.noiframes+"</iframe><div id='TB_overlay'></div><div id='TB_window' class='thickbox-loading'></div>");
 				jQuery("#TB_overlay").click(tb_remove);
 			}
 		}else{//all others
 			if(document.getElementById("TB_overlay") === null){
-				jQuery("body").append("<div id='TB_overlay'></div><div id='TB_window'></div>");
+				jQuery("body").append("<div id='TB_overlay'></div><div id='TB_window' class='thickbox-loading'></div>");
 				jQuery("#TB_overlay").click(tb_remove);
 				jQuery( 'body' ).addClass( 'modal-open' );
 			}
@@ -260,7 +267,7 @@
 //helper functions below
 function tb_showIframe(){
 	jQuery("#TB_load").remove();
-	jQuery("#TB_window").css({'visibility':'visible'});
+	jQuery("#TB_window").css({'visibility':'visible'}).trigger( 'thickbox:iframe:loaded' );
 }
 
 function tb_remove() {