Adapted from: http://fluxbox-wiki.org/Make_windows_only_show_a_border.html

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#  HOW TO MAKE WINDOWS ONLY SHOW A BORDER  #
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1. Click Settings > Configure > Init (~/.fluxbox/init), find the related entry with a search on "deco" and change its value from NORMAL (the default) to BORDER:

session.screen0.defaultDeco: BORDER

Now save and click Leave > Refresh from the rootmenu. All apps will show ONLY a border now (if the style uses one). 

2. You can add a small tab to your BORDER config as described in 1. above, by editing your ~/.fluxbox/init file like this:

session.screen0.defaultDeco: TAB

You will get a window with border (as described above) but with a nice tab attached as configured in Settings > Configure > Window > Tab options.

3. You can also change the pixelwidth and the color of the border.

Either do it globally using the overlay file or separately for every single style. In both cases you need to add three lines to the appropriate file:

window.borderWidth:   
window.focus.borderColor: 
window.unfocus.borderColor: 

4. Video: https://youtu.be/LUwKZXH8xL0
