Adjust the position of modal dialogues

The top was off the top of the window

themes/tiddlywiki/vanilla/base.tid

@@ -766,7 +766,7 @@ canvas.tw-edit-bitmapeditor  {
 		top: 50%;
 		left: 50%;
 		width: 560px;
-		margin: -250px 0 0 -280px;
+		margin: -180px 0 0 -280px;
 	}
 }