How do I work with (minimize/resize/un-float) a captive portal?
Because of System Integrity Protection (SIP), a feature introduced in El Capitan to protect system files from modification by malware, you can’t just move the file if you have that feature enabled (it’s on by default). 

So, first, you have to disable SIP on this case

Follow these steps to disable SIP:


Restart your Mac.
Before OS X starts up, hold down Command-R and keep it held down until you see an Apple icon and a progress bar. Release. This boots you into Recovery.
From the Utilities menu, select Terminal.
At the prompt type exactly the following and then press Return: csrutil disable
The terminal should display a message that SIP was disabled.
From the Apple menu, select Restart.


After this, 


In the Finder, select Go > Go To Folder.
Enter /System/Library/CoreServices and hit return.
Find Captive Network Assistant, click it, and rename it with an extra word, like Captive Network Assistant Do Not Launch and press return.
Enter your password when prompted to make the change.


After this again, enable SIP, following the first process shared on this comment just by typing csrutil enanble