1407777 - Jank when downloading a file to a network drive

Status: NEW

(Firefox :: File Handling, enhancement, P3)

Description

[(not currently active) Ted Mielczarek]

My default download directory on my Windows machine is a network drive. I downloaded a file today and the browser UI hung for several seconds. Thankfully I had the profiler running, so I captured a profile:
https://perf-html.io/from-addon/calltree/?hiddenThreads=&invertCallstack&thread=0&threadOrder=0-2-3-4-5-7-1-6&transforms=f-combined-zN1zN0zMvzMuzMtzMszMrPvPuPtxDiADkyKazMq-i

It looks to be hung stat'ing the download directory on the main thread here:
https://dxr.mozilla.org/mozilla-central/rev/a0488ecc201c04f2617e7b02f039344e8fbf0d9a/toolkit/mozapps/downloads/nsHelperAppDlg.js#24

By way of validateLeafName called from here:
https://dxr.mozilla.org/mozilla-central/rev/a0488ecc201c04f2617e7b02f039344e8fbf0d9a/toolkit/mozapps/downloads/nsHelperAppDlg.js#263

The call happens from inside an async function, so it shouldn't be hard to make this use `OS.File.stat` to do the file access off the main thread.