RadImageGallery fullscreen mode overrides viewport meta tag set on page.
If the user sets the following meta tag:

<meta name="viewport" content="width=device-width, initial-scale=1.0, minimum-scale=1, maximum-scale=1.0, user-scalable=no, height=device-height" />

initially the page is not scalable. When the Gallery enters fullscreen mode, the page is zoomed out.
And when you exit fullscreen, the page stays zoomed out, the gallery proportions are changed and fonts are smaller.

Will work in Q2 2014

Creating a large page with RadEditor control and opening the mentioned dialog will scroll the page to the top. This causes the user to loose visual  contact with the Editor.

The issue is due to the automatic textarea focusing in the clientInit() method of the dialog's logic.

To resolve the issue you should modify the dialog and remove the focusing functionality. To do so follow the next steps:

    1. Follow the Custom Built-in dialogs approach (http://demos.telerik.com/aspnet-ajax/editor/examples/externaldialogspath/defaultcs.aspx) to use an external and later modified Plain Paste Dialog - the MozillaPasteTextDialog.ascx;
    2. Find the clientInit() method trough the client-side logic of the MozillaPasteTextDialog.ascx file;
    3. Remove the this._container.focus() line.

You can also find attached a sample page with the fix placed, so that you can examine it.
 

The issue is caused by a bug in the KB3057839 windows update. The bug has been reported to Microsoft and can be tracked at the following link:

https://connect.microsoft.com/VisualStudio/feedback/details/1475291

Currently the only workaround is to revert the KB3057839 update on the production server.