We've had that in the past but decided to remove it. Instead, use quick open to open files:

Ctrl + Alt + O -> type -> enter

the pattern syntax is quite advanced and works with substrings etc. Once you get used to it, you shouldn't have a need to use the project toolview that much anymore. Example:

pl/qui/test_ -> plugins/quickopen/test_quickopen.cpp
duchain/ident -> language/duchain/identifier.cpp

etc. pp.

Hi Milian,

I couldn't find the relevant email thread in kdevelop-devel@, can you please share the link/e-mail subject?

Quick Open is nice, but sometimes it's harder to use: when I need to open a few similarly named files from different directories (e.g. abc.h, abc.cpp, abc.ui, etc),
 - With Quick Open I have to type in this name over and over again for each of those files and then look for the needed files in the search results;
 - With the File Tree Filter Box I could type in the name ("abc") once and then double-click a few times to open all the necessary files.

Also, when there are several open projects in KDevelop session, Quick Open shows relevant files from all of those projects as a flat list, it's then hard to look for files from only one selected project because files from the other projects are mixed in that list.

I don't know the email thread either, maybe we just discussed it on IRC. The commit that removed the filter is:

commit 8c7a559f9d92cd8a4d3ea32eba8a19643a47613a
Author: Milian Wolff <mail@milianw.de>
Date:   Tue Nov 5 14:57:01 2013 +0100

    Remove proxy-model filtering from ProjectManagerView/ProjectProxyModel
    
    This is now handled in a much more efficient way via the ProjectFilter
    plugin. The dynamic filtering in the project view is thus obsoleted.
    
    Thus: If you want to exclude stuff from your project, use the filter
    plugin. If you want to quickly open a file for a given pattern, use
    quickopen.

Regarding your workflow: Note that there is an action to focus the current file in the project tree view. So for your case, you can just open one of the files, e.g. abc.cpp, then activate the action (in my case with Ctrl + <) and open the files.

Furthermore note that QuickOpen is smart enough to resolve "./" to filter based on the path of the currently opened file, giving you another quick way to open the sibling files.