Firefox 83.0 RC 2 and latest 84.0a1 on Windows 8.1

Hitting a breakpoint set earlier with the debugger not loaded yet this time opens the debugger tab but doesn't list source files, breakpoints and source code.

Steps to reproduce:

1. Load https://charts.mozilla.org/NeglectedOranges/index.html
2. Open the debugger (Ctrl+Shift+Z)
3. In the left sidebar, expand and select: charts.mozilla.org > Neglected Oranges > index.html
4. Set a breakpoint in line 99 (Log.actionDone(a)) by clicking on the line number.
5. Open a new tab.
6. Close the old tab.
7. In the new tab, open the developer tools a non-Debugger tab (don't seelect the Debugger one), e.g. Network (Ctrl+Shift+E).
8. Load https://charts.mozilla.org/NeglectedOranges/index.html

Actual result:
Breakpoint hit, script execution halted, Debugger tab focused but left sidebar doesn't list resources, script pane doesn't show source code with breakpoint hit and right sidebar also doesn't list breakpoints.

Expected result:
Breakpoint hit, script execution halted, Debugger tab focused, left sidebar lists resources, script pane shows source code with breakpoint hit and right sidebar also lists breakpoints.

Thanks a lot for the report!

I don't reproduce this particular behavior, I don't reproduce the missing sources.
Instead, I don't see the breakpoint being hit at all :/
The debugger isn't focused, nor highlighted in green, I have to reload the page in order to make it work. Then it works fine.
I reproduce this quite easily on any website.
The breakpoint isn't hit at all as the website keeps running.

The breakpoint isn't hit at all as the website keeps running.

What's what I observed for 82.0.3, but directly before bug 1620280 landed it worked as expected during the bisection.

It looks rather related to cross process navigation. When you open a new tab, and you are having about:newtab, this is loaded in a special process, which is different from the webpage your are loading (chars.mozilla.org). That without and with fission, the behavior is the same.

Before bug 1663649, the devtools were closing and reloading, making it slightly more obvious that things may be brittle when doing such navigation. After this bug, the devtools are kept open, but the support of breakpoints is still brittle, that, until bug 1573327 will hopefully land soon.

Tracking dt-fission-m3-mvp bugs for Fission MVP milestone

This might be a duplicate of bug 1678254. Sorry if I did not spot the right STR sooner. Greg helped me find something I reproduced.
This probably has been fixed by bug 1644188, which landed today.

Sebastian can you confirm the fix?

Verified fixed with Firefox 85.0a1 20201209034903.