By modifying the code jQuery uses to implement the alsoResize option, we can make our own alsoResizeReverse option. Then we can simply uses this as follows:
$("#resizable").resizable({
alsoResizeReverse: ".myframe"
});
And the code for adding the alsoResizeReverse option: Only a few things had to be changed, such as the obvious renaming alsoResize to alsoResizeReverse and subtracting the delta instead of adding it (what makes the resize reversed). The original alsoResize code starts on line 2200 in this version of jQuery UI (1.8.1 from the Google CDN).
The code (this should be put outside of document.ready()):
$.ui.plugin.add("resizable", "alsoResizeReverse", {
start: function(event, ui) {
var self = $(this).data("resizable"), o = self.options;
var _store = function(exp) {
$(exp).each(function() {
$(this).data("resizable-alsoresize-reverse", {
width: parseInt($(this).width(), 10), height: parseInt($(this).height(), 10),
left: parseInt($(this).css('left'), 10), top: parseInt($(this).css('top'), 10)
});
});
};
if (typeof(o.alsoResizeReverse) == 'object' && !o.alsoResizeReverse.parentNode) {
if (o.alsoResizeReverse.length) { o.alsoResize = o.alsoResizeReverse[0]; _store(o.alsoResizeReverse); }
else { $.each(o.alsoResizeReverse, function(exp, c) { _store(exp); }); }
}else{
_store(o.alsoResizeReverse);
}
},
resize: function(event, ui){
var self = $(this).data("resizable"), o = self.options, os = self.originalSize, op = self.originalPosition;
var delta = {
height: (self.size.height - os.height) || 0, width: (self.size.width - os.width) || 0,
top: (self.position.top - op.top) || 0, left: (self.position.left - op.left) || 0
},
_alsoResizeReverse = function(exp, c) {
$(exp).each(function() {
var el = $(this), start = $(this).data("resizable-alsoresize-reverse"), style = {}, css = c && c.length ? c : ['width', 'height', 'top', 'left'];
$.each(css || ['width', 'height', 'top', 'left'], function(i, prop) {
var sum = (start[prop]||0) - (delta[prop]||0); // subtracting instead of adding
if (sum && sum >= 0)
style[prop] = sum || null;
});
//Opera fixing relative position
if (/relative/.test(el.css('position')) && $.browser.opera) {
self._revertToRelativePosition = true;
el.css({ position: 'absolute', top: 'auto', left: 'auto' });
}
el.css(style);
});
};
if (typeof(o.alsoResizeReverse) == 'object' && !o.alsoResizeReverse.nodeType) {
$.each(o.alsoResizeReverse, function(exp, c) { _alsoResizeReverse(exp, c); });
}else{
_alsoResizeReverse(o.alsoResizeReverse);
}
},
stop: function(event, ui){
var self = $(this).data("resizable");
//Opera fixing relative position
if (self._revertToRelativePosition && $.browser.opera) {
self._revertToRelativePosition = false;
el.css({ position: 'relative' });
}
$(this).removeData("resizable-alsoresize-reverse");
}
});
Here's a demo: http://jsfiddle.net/WpgzZ/