By design RadEditor does not support size in percentages and this is why its size is not reset when browser window is resized.
The only possible approach I could suggest you is to implement your all Client-Side logic on the Editor's load and the window resize events. In this function you could take the current size of the browser window and set it as a width of the RadEditor control by using its setSize()
method. You can do it in a similar way:
window.onresize = resizeEditor;
editor = $find(
//set the width and height of the RadEditor
windowWidth = $telerik.$(window).width();
editorHeight = $telerik.$(
editor.setSize(windowWidth - 20, editorHeight);
Note, that if the initially set width of the Editor is configured in percentages the code above will not work. I hope this helps.
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