The X is much more eye catching than the collapse arrows, so users accidentally close the project filter (intending to clear the filter) and don't know how to make it visible again. This is our most frequently asked question and it occurs at least once in every training I have given (I trained around 150 users in our company).
I suggest to remove the x button from the UI (or alternatively redesign its functionality so that it clears the filter). This would make it more consistent with other views.
And I would simplify the collapse/expand arrows to one single collapse/expand button and make it bigger. They are really small on high-res display. This is of course valid in other views as well, like the outline pane in the document view, the item details preview, etc.