That was Chrome 12.0.742.30 (84361) on Ubuntu 11.04. Firefox 4.01 and Opera 11.10 will produce identical rendering; but I'm not even discussing if the components should be rendered vertically or horizontally; instead their widths seem to be wrong.
Sorry I guess I am not sure what the problem is you are referring to. Every browser I have tried (Win7 Chrome 11/ FF 4/ IE 9) renders a fieldset's items vertically by default. Fieldsets have an anchor layout by default which you are not using in your example.
As for fieldset items, each field's width appears to be the combination of the label and the form field. You can change the size of the field label using the labelWidth config option on the field.
width - labelWidth = size of textbox/combobox in your example
The problem here is: outside a FieldSet, [width] refers to the component width, as expected; [labelWidth]s are calculated separately. Inside a FieldSet, [width] refers to [labelWidth] + component width. That is very confusing. Is that even documented anywhere ? On 3.x that didn't happen.
As for the combo properties, you're right, I'm porting code from 3.x and didn't finish cleaning it up, but that does not have any influence on the example.