Flyspray

Interesting, but implementation of that search filter probably not before FS1.1.

Would need some even more complicated sql query building in get_task_list()..

A maybe simpler modification would be to show just the reason of closing after the 'closed' status in the task list (if 'status' column is shown).

UI extendend search form: By default hide a resolution dropdown/select. Only if the search status 'closed' is selected - and only this one option!, then show the resolution dropdown for sub filtering.

I currently do not see how the hiding/showing here can be done with pure HTML/CSS without Javascript, so for javascript disabled browsers the resolution dropdown is shown always in extended search form and hidden by javascript on page load.

Maybe this for pure HTML/CSS:

<select id="status">
..
<option id="closed" value="closed">closed</option>
<option id="reason1" value="r1">- Reason 1</option>
<option id="reason2" value="r2">- Reason 2</option>
..
<option value="3">other status</option>

and

<style>
option#reason1,option#reason2, ..  {display:none;background-color:#eee;} /* or regexp */
option#closed:checked:before {
content:"Filter by closing reasons with CTRL+SPACE or CTRL+Mouse, navigate with up/down keys";
position:absolute;
top:-50px;
background:rgba(200,200,200,0.8);
left:-40px;
color:#900;
}
option#closed:checked ~ option#reason1,option#closed:checked ~ option#reason2, {display:block;} /* or regexp */
</style>