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Rendering of the left sources panel got broken somewhere between gtk 3.10 and gtk 3.14, as some treeview rendering mechanisms might have changed: now OnRender is not called anymore when the width is 0; so let's call the base implementation of OnGetPreferredWidth() here instead. This fixes the main glitch about using Banshee 2.9.x in Ubuntu 15.04, for example, which already bundles gtk 3.14.x.
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