tinymce_sample.js

editor.windowManager.open( {
					title: 'A popup dialog',
					body: [
						{
							type: 'textbox',
							name: 'text1',
							label: 'enter your text',
							multiline : true,
							minHeight : 60,
							
						},
						{
							type:  'container',
							html: String( text1.length() ),
							label: 'This changes dynamically based on length of text entered above',
						}
					],

I've also tried this.text1.length() and str.length(text1) among other things. Is what I am trying to do even possible?

I added an onchange: function() {...} block to the end of the textbox to declare a variable that I then call within the html: String() function. Not working yet.

I think you need to have the function to count outside of the window manager, and call it, targeting the class or ID of the text field

and then call that function inside of the html: block, or in the textbox with an onchange: or something?

I'm gonna put my actual plugin code with the new stuff up here, so that it becomes clearer.