STRs:

• add debugger; at https://searchfox.org/mozilla-central/source/devtools/client/inspector/animation/test/browser_animation_animated-property-list.js#22
• run ./mach mochitest devtools/client/inspector/animation/test/browser_animation_animated-property-list.js --jsdebugger

ER: Should hit the breakpoint, open the source in the debugger etc...
AR: The debugger looks paused (green icon) but no source is displayed

Note, it doesn't reproduce 100% of the time. For me it fails 90% of the time, but sometimes I still manage to get a correct debugger UI.

Note: the line and file don't matter too much I think.
I also get the same issue when I try to set breakpoints in actual implementation modules (devtools modules, JSMs etc...)

Might be related to Bug 1676618

(In reply to Julian Descottes [:jdescottes] from comment #0)

Note, it doesn't reproduce 100% of the time. For me it fails 90% of the time, but sometimes I still manage to get a correct debugger UI.

I'm not able to repro from my linux VM. It probably only reproduces on fast hardware :/

I tried several times and it works for me (Win10, m-c, fast machine)

The MBT window would deserve to be focused and placed on top when the BP hits, otherwise it's so easy to miss it since it's hidden behind the (debugee) Firefox window.

Thanks for testing! Interesting that I seem to be the only one facing this. It might be platform specific.
Nicolas, could you try the STRs from the summary on macos ?

Doing the steps from Comment 0, I was able to hit the breakpoint and seeing the source 5 times out of 5, on latest central.

Interesting. On a clean profile latest central, I still get the same issue. Works once in a while (1 time in over 10 tries today).

Will record my steps in case I do something odd which I don't realize.

The screenrecording was taken after a clobber + (artifact) build on top of latest central (https://hg.mozilla.org/mozilla-central/rev/d9e30da70def for me here).

I did a few other attempts after that, trying to click faster/slower on the test window to start the tests. Always got the same result.

Quick round of needinfos: Can you watch the screen recording and let me know if you see anything odd?

Tried to reproduce it again today, doing the same exact steps as you do, moving windows a bit, but the source always shows up.

Tried again today (4 times) and it works for me.

Julian, does it help if you create a new profile?

I always use a clean profile, after a clobber build. So no it's not profile related. At this point I suppose there's something odd with my machine & or my OS.

(edit: also when you run mach test, you always use a temporary profile)

Tried again today, I still get the issue, but it seems a bit less frequent.

I'm running into the same issue, with different source files (toolkit/components/normandy/). I'm on Linux, also using artifact builds. I get this 100% of the times I've tried. It is very repeatable for me, and is getting in the way of development. My version of it looks slightly different, as I don't have any sources available in the left panel.

I think the fix on Bug 1720184 should also address this, and it's probably the same root cause which only became more frequent for some reason.