And there can be easily reproduced in examples provided within ExtJs.
1. Put extjs package to a web-container.
2. Open ext\examples\tree\two-trees.js, at the line 21 of the file, modify the width value to 100.
3. Use IE8 browser to view the "two trees" example. (such as http://localhost/ext/examples/tree/two-trees.html)
4. Expand the left tree and click around the leaves.
Few days back I ran into the same issue while working with Ext 4.1.2 version. My tree panel nodes flick/flash before expanding, when I click on the plus/minus icon. This is happening only with IE8,working on Windows 7.
Other bowsers make the expand/collapse animations smooth.