When this property is set to true, a preload progressbar is displayed while the editor's images are loading.
If this property is set to true, a progress bar is displayed while images used by the WebHtmlEditor are loading. The progress bar will only be displayed for the first request. For any subsequent postbacks, this property will be set to false, so the progress bar will not be displayed.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2