source: Dev/trunk/src/client/dijit/form/_ExpandingTextAreaMixin.js

define([
        "dojo/_base/declare", // declare
        "dojo/dom-construct", // domConstruct.create
        "dojo/has",
        "dojo/_base/lang", // lang.hitch
        "dojo/on",
        "dojo/_base/window", // win.body
        "../Viewport"
], function(declare, domConstruct, has, lang, on, win, Viewport){

        // module:
        //              dijit/form/_ExpandingTextAreaMixin

        // feature detection, true for mozilla and webkit
        has.add("textarea-needs-help-shrinking", function(){
                var body = win.body(),  // note: if multiple documents exist, doesn't matter which one we use
                        te = domConstruct.create('textarea', {
                        rows:"5",
                        cols:"20",
                        value: ' ',
                        style: {zoom:1, fontSize:"12px", height:"96px", overflow:'hidden', visibility:'hidden', position:'absolute', border:"5px solid white", margin:"0", padding:"0", boxSizing: 'border-box', MsBoxSizing: 'border-box', WebkitBoxSizing: 'border-box', MozBoxSizing: 'border-box' }
                }, body, "last");
                var needsHelpShrinking = te.scrollHeight >= te.clientHeight;
                body.removeChild(te);
                return needsHelpShrinking;
        });

        return declare("dijit.form._ExpandingTextAreaMixin", null, {
                // summary:
                //              Mixin for textarea widgets to add auto-expanding capability

                _setValueAttr: function(){
                        this.inherited(arguments);
                        this.resize();
                },

                postCreate: function(){
                        this.inherited(arguments);
                        var textarea = this.textbox;
                        textarea.style.overflowY = "hidden";
                        this.own(on(textarea, "focus, resize", lang.hitch(this, "_resizeLater")));
                },

                startup: function(){ 
                        this.inherited(arguments);
                        this.own(Viewport.on("resize", lang.hitch(this, "_resizeLater")));
                        this._resizeLater();
                },

                _onInput: function(e){
                        this.inherited(arguments);
                        this.resize();
                },

                _estimateHeight: function(){
                        // summary:
                        //              Approximate the height when the textarea is invisible with the number of lines in the text.
                        //              Fails when someone calls setValue with a long wrapping line, but the layout fixes itself when the user clicks inside so . . .
                        //              In IE, the resize event is supposed to fire when the textarea becomes visible again and that will correct the size automatically.
                        //
                        var textarea = this.textbox;
                        // #rows = #newlines+1
                        textarea.rows = (textarea.value.match(/\n/g) || []).length + 1;
                },

                _resizeLater: function(){
                        this.defer("resize");
                },

                resize: function(){
                        // summary:
                        //              Resizes the textarea vertically (should be called after a style/value change)

                        var textarea = this.textbox;

                        function textareaScrollHeight(){
                                var empty = false;
                                if(textarea.value === ''){
                                        textarea.value = ' ';
                                        empty = true;
                                }
                                var sh = textarea.scrollHeight;
                                if(empty){ textarea.value = ''; }
                                return sh;
                        }

                        if(textarea.style.overflowY == "hidden"){ textarea.scrollTop = 0; }
                        if(this.busyResizing){ return; }
                        this.busyResizing = true;
                        if(textareaScrollHeight() || textarea.offsetHeight){
                                var newH = textareaScrollHeight() + Math.max(textarea.offsetHeight - textarea.clientHeight, 0);
                                var newHpx = newH + "px";
                                if(newHpx != textarea.style.height){
                                        textarea.style.height = newHpx;
                                        textarea.rows = 1; // rows can act like a minHeight if not cleared
                                }
                                if(has("textarea-needs-help-shrinking")){
                                        var     origScrollHeight = textareaScrollHeight(),
                                                newScrollHeight = origScrollHeight,
                                                origMinHeight = textarea.style.minHeight,
                                                decrement = 4, // not too fast, not too slow
                                                thisScrollHeight,
                                                origScrollTop = textarea.scrollTop;
                                        textarea.style.minHeight = newHpx; // maintain current height
                                        textarea.style.height = "auto"; // allow scrollHeight to change
                                        while(newH > 0){
                                                textarea.style.minHeight = Math.max(newH - decrement, 4) + "px";
                                                thisScrollHeight = textareaScrollHeight();
                                                var change = newScrollHeight - thisScrollHeight;
                                                newH -= change;
                                                if(change < decrement){
                                                        break; // scrollHeight didn't shrink
                                                }
                                                newScrollHeight = thisScrollHeight;
                                                decrement <<= 1;
                                        }
                                        textarea.style.height = newH + "px";
                                        textarea.style.minHeight = origMinHeight;
                                        textarea.scrollTop = origScrollTop;
                                }
                                textarea.style.overflowY = textareaScrollHeight() > textarea.clientHeight ? "auto" : "hidden";
                                if(textarea.style.overflowY == "hidden"){ textarea.scrollTop = 0; }
                        }else{
                                // hidden content of unknown size
                                this._estimateHeight();
                        }
                        this.busyResizing = false;
                }
        });
});