Version:           2.0.90 (using KDE 4.2.85)
OS:                Linux
Installed from:    Ubuntu Packages

Amarok changes its pane width arbitrarily, to me it seems it's the middle pane that consumes that space and is the culprit. I use amarok with a wide playlist pane (amarok1 layout) and the middle pane as left pane replacement. 

When I open the left pane with the collection I give it a size in which the collection entries are readable (wider than the default width), without changing the width of the playlist so that the middle pane gets shrinked to minimum when the left pane is open. When I close the left pane the middle pane goes back to the size I want. But when I close amarok and reopen it and reopen the left pane it is narrow again and the middle pane is wider - wider to the left and wider to the right. 

I expect that (unless the window size changes) amarok keeps the borders between the panes fixed once set by the user (until he changes it). The right playlist pane width shouldn't change at all. And the left collection pane should also keep its width. The middle pane should adapt its width: when I open the left pane it should shrink without touching the width of the right playlist pane, when I close the left pane it should go back to its original user set width also without touching the right pane's width.

When the window size changes (manual or per maximize) all panes should expand the same percentage of width according to their use set size in normal window mode.