Unanswered: Grid columns broken in some browsers when num columns high
As the number of columns in your grid increases the columns list (accessed via the dropdown arrow on the column header > Columns) fails to render properly. See the attached "Chrome+Firefox-bug.png" in the zip. NB this bug varies depending on which browser you use - IE 9 still renders the list ok (see "IE-ok.png").
I've attached a simple example (NB this problem is affecting a real grid in my production code, I'm not just being pedantic). To make the example I simply duplicated the first column in one of the Ext-JS examples (ext-4.0.2a\examples\grid\array-grid.js) a couple times till the bug appeared. There are 24 columns in the example grid - chrome and firefox don't render the Columns list properly, IE is ok. If you delete one (ie 23 cols) then the column list is rendered properly in all browsers. In my production code the problem occurred after 13 columns.
Unzip the file to ext-4.0.2a\examples\ - it will create: