The images below show our application using 778 and 825. When using 825 (or 803) the tree grids show a horizontal scroll bar. The reason appears to be that the widths specified in the <colgroup><col/><colgroup> are each 1 px too wide. That is, the difference between the generated html for the two ExtJS versions is a difference of 1 pixel in the width style attribute of each <colgroup>. If these values are changed in 825 generated html using the debugger the horizontal scroll bar disappears.
Let me know if you need a test case. We have other tree grids in which the problem does not arise so there may be something about this specific layout (two grids on the same panel maybe?). It'll take a while to pries the application apart to preserve the error so if you have an existing test it would save the effort.