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This is an acknowledged bug in libwnck with many duplicates. It takes so long because the Gnome folks are pondering how to fix it The Right Way(TM). Though I personally think that the patch should be dead simple (set a fixed button size, possibly depending on screen resolution). Somewhere there even is a patched libwnck .deb for Edgy that has this annoyance fixed.
I just tried out fiesty Herd5 live CD and it seemed to be fixed in that. :-) I am guessing ubuntu has patched this and not actually fixed in Gnome. Edited 2007-03-15 00:29
Then I might not have understood correctly the first comment above. My firefox entry in the taskbar still resizes depending on the Title size.
It takes so long because the Gnome folks are pondering how to fix it The Right Way(TM).
Set a maximum button width based on the width of the taskbar widget such that a user-configurable whole number of buttons fits in the taskbar at this maximum width. If the number of buttons is higher than this threshold, each button is sized such that they all have the same width. Button width is only recalculated when adding or deleting buttons from the taskbar or resizing the taskbar.
That seems like reasonable and pretty simple behavior to me, but then again, my GUI programming experience isn't very extensive.