The patch allows the user to open and close the search field in the outliner header.
This would help to keep the editor smaller in simple scenes, where the search function is not frequently used, while keeping the "filter" and "add collection" buttons visible.
Knowing nothing about C flags and bit to bit operations, after lots of failures in making this work I've searched for one of the flags and so, as you'll notice @Dalai Felinto (dfelinto) , this is basically a revert of this commit of yours: https://developer.blender.org/rB0145daa09bba74df474f0626fac8372c0a7e2b90
Even if this clearly has already been discussed (during the code quest I guess), I wanted to try and post this anyway, as I really think that it would make the outliner way more easy to use in simple scenes, with very few space added when expanded.
Current vs. proposed (closed):
Current vs. proposed (open):