STRs: 1. Set dom.ipc.processCount to 1 in about:config 2. Go to https://mozilla.github.io/webrtc-landing/gum_test.html and press the button screen 3. Open another tab, repeat step 2. Expected behavior: Firefox shows the permission prompt. Actual behavior: No prompt is shown.

mozregression result point to the bug 1371000.

I can reproduce this bug on Windows 7 and Windows 10. Different from bug 1399413, this bug occurs when the 2nd gUM desktop sharing is runned by the same content process as the 1st gUM desktop sharing. When the issue occurs, VideoCapture thread stucks at Windows api EnumWindows [1][2] and never returns. [1] http://searchfox.org/mozilla-central/rev/dca019c94bf3a840ed7ff50261483410cfece24f/media/webrtc/trunk/webrtc/modules/desktop_capture/window_capturer_win.cc#132 [2] https://msdn.microsoft.com/zh-tw/library/windows/desktop/ms633497(v=vs.85).aspx

Actually, VideoCapture thread hangs at winapi GetWindowTextLength/ [1] [1] http://searchfox.org/mozilla-central/rev/dca019c94bf3a840ed7ff50261483410cfece24f/media/webrtc/trunk/webrtc/modules/desktop_capture/window_capturer_win.cc#35 I move the snippet calling GetWindowTextLength and the hang sympton just gone.

It seems a winapi bug. Calling GetWindowTextLength() with an invisible window handle would cause hang. Although I don't find any discussion about this.

Comment on attachment 8920077 [details] Bug 1409643 - call GetWindowTextLength() for visble window only. https://reviewboard.mozilla.org/r/191084/#review198292

(In reply to Munro Mengjue Chiang [:mchiang] from comment #0) > Actual behavior: > No prompt is shown. To clarify, "no prompt" means that A) nothing happened, or B) prompt was bypassed and sharing worked? I assume A.

(In reply to Jan-Ivar Bruaroey [:jib] (needinfo? me) from comment #7) > (In reply to Munro Mengjue Chiang [:mchiang] from comment #0) > > Actual behavior: > > No prompt is shown. > > To clarify, "no prompt" means that A) nothing happened, or B) prompt was > bypassed and sharing worked? I assume A. Yes, VideoCapture thread will hang, and anything behind will not happen.

Pushed by mchiang@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/ad3d44a5fa1e call GetWindowTextLength() for visble window only. r=jib