This was happening for a very long time now, but I always forgot to ask about it.
Sometimes, the sidebar’s width on the Visor may reset (unsure if this happens with normal windows, as I almost always use the Visor).
Consider the following scenario (narrowed sidebar width):
in which afterwards, the visor is closed and one continues doing tasks, and then they double click an external hard drive icon on the desktop, “500GB”, which may yield this result:
Of course, after closing this tab, one gets:
but if the sidebar were to be re-resized back, then after closing this tab, the width would remain narrow:
(OSX 10.9.5 on a mid14 rMBP 15")