Merely setting the AutoSize property of the RadWindow to true is not always sufficient for this functionality to work as expected. There are several other aspects that need to be taken into account:
The content placed in the RadWindow needs to have explicit dimensions set in pixels. This is necessary, as otherwise you get a confrontation of concepts - a block element (such as the asp Panel, or div, or table) by default takes the width of its parent - 100% of it. This means that it tries to autosize according to the RadWindow. In the same time we want the RadWindow to autosize itself according to this element, which leads to this obvious contradiction. This is the reason why some explicit dimensions must exist in the content. Therefore one of the best solutions is adding an empty div with no content or borders that has explicit width - it will not be visible, yet it will allow the RadWindow to autosize correctly.
The autosizing feature is designed to work with the XHTML 1.0 transitional doctype and it is recommended that it is used for the page containing the RadWindow:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "https://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
Other doctypes, such as the HTML strict have some limitations on the way elements are positioned and sized and autosizing may not work correctly with them.
The page containing the RadWindow also needs to be valid HTML, as well as the content itself, as errors in either (such as extra semicolons in properties, tags left unclosed, incorrectly closed tags, etc.) result in the page not rendering properly and therefore the RadWindow may not be able to properly calculate the needed dimensions. You can easily confirm this by using the W3C Validator.
There may be some issues with calculating dimensions when the top-level containers have margins or borders, so they may not be taken into account when calculating the size of its content. This also applies for paddings, as nesting elements when the parent has padding has similar effect. Therefore a good approach is to have a main container that has no borders, margins and paddings, yet has its dimensions set (or at least width). This also has to be extended to the html, body and form elements when an external page is loaded - i.e. that also need the paddings and margins removed:
html, body, form
If you get scrollbars (e.g., after minimize/maximize/restore) or the size is off by a few pixels, adding the following rule can remove them. If the size is off by 1px, this will often add one scrollbar (for example, vertical), which will, in turn, take up some space and cause a horizontal scrollbar. Such an issue with the size can come from rounding of the RadWindow size - HTML is shown (rendered) by the browser in integer values for the pixels, but may be calculated in decimal values because of the
em units used by the CSS. This provides margin for rounding that can cause such issues.
When using the AutoSize functionality setting explicit dimensions is not necessary. Moreover, they can even result in incorrect dimensions. For example, if you set the MaxHeight and MaxWidth properties to values smaller than the dimensions that will actually be needed by the content the RadWindow's size will be limited and scrollbars will be spawned. The situation with MinWidth and MinHeight is similar - if they are larger than the content you will usually get extra white space.