jQuery returns incorrect window height

I have a site (it's an intranet, so I can't give a link) which uses jQuery.ui dialogs to show messages.
It uses only jQuery and jQuery.UI plus some of our own JS and CSS.
When the page content is much bigger than the view port (the browser window) (so there is vertical scroll bar appears) the dialog is shown at the bottom of the browser's window. Also the opposite is true, when the page content is much less to the browser's window, the dialog is aligned to the top edge.
I've found out that on this site the following expression is always equals true (in Chrome and Mozilla, but not in IE) and I believe that is the reason for the dialog misplacing.

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On other sites these two methods return different sizes of the page content and the viewport accordingly.
What could be a cause for that?
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Dave BaldwinFixer of ProblemsCommented:
For what it's worth, this page has all the info too:  http://www.dibsplace.com/webdev/browserinfo.html   Feel free to copy it if you want.
Julian HansenCommented:
The selected answer is a blind link - here is the recommended solution from that page.

If the page is missing the <!DOCTYPE> declaration - jQuery will return a document height that is equal to the window height.

Adding the <!DOCTYPE html> declaration to the top of the page should fix this issue.

From the page https://viralpatel.net/blogs/jquery-window-height-incorrect/
Our HTML document doesn’t have DOCTYPE declaration. So it’s not a valid HTML so to speak. JQuery cannot calculate window height / document height correctly if doctype is not specified on window.

The <!DOCTYPE> declaration is not an HTML tag; it is an instruction to the web browser about what version of HTML the page is written in.
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