The tree view in Log/History tab cannot be resized. I see the resize mouse pointer on the separator bar between the tree view and the commit-detail/diff view but when I click and drag a slight jerk in the size occurs and then nothing. The height is fixed at half the available area. I am able to resize the bookmarks, branch, commit details, file list and diff views.

Also l would like to add that SourceTree is the best tool I have used till now for Git. I really like the clean UI and git-flow integration.