STR:

1. Ctrl+Shift+J to open the Browser Console.

2. Do something that triggers a JS error (red highlighted entry) with a collapsible/uncollapsible backtrace in the Browser Console. (i.e. a little triangle icon that can be clicked to show/hide the backtrace).
   For example, you could perform bug 1523020's STR in current Nightly -- the first three errors shown in the screenshot there are the type of error that I'm concerned with here:
   https://bug1523020.bmoattachments.org/attachment.cgi?id=9039272

3. Click the triangle icon to expand the backtrace (so it's visible as in that^^ screenshot)

4. Try to copy & paste the backtrace into a bug comment field or into a text editor.

EXPECTED RESULTS:
The backtrace should paste in as a multi-line text expression, with a newline character at the end of each backtrace level.

ACTUAL RESULTS:
The backtrace is pasted in all on a single line, with no newline characters, even though it showed up as a nicely formatted multi-line thing in the Browser Console.

This makes it pretty annoying when trying to post a backtrace into a bug report, for example. (I was going to do this on bug 1523020 but ended up posting a screenshot instead because the formatting was broken enough to make it unreadable if I copypasted.)

E.g. if I try to paste in the first error from that screenshot (including its backtrace) into this bug comment, here's what the pasted output looks like:

Error while calling actor 'parentProcessTarget's method 'ensureCSSErrorReportingEnabled' Argument 1 of InspectorUtils.getAllStyleSheets is not an object.
protocol.js:1048:5

writeError resource://devtools/shared/protocol.js:1048 handler resource://devtools/shared/protocol.js:1243 handler resource://devtools/shared/protocol.js:1243 _queueResponse resource://devtools/shared/protocol.js:1062 handler resource://devtools/shared/protocol.js:1242 onPacket resource://devtools/server/main.js:1279 send resource://devtools/shared/transport/local-transport.js:64 makeInfallible resource://devtools/shared/ThreadSafeDevToolsUtils.js:109 makeInfallible resource://devtools/shared/ThreadSafeDevToolsUtils.js:109 

Daniel, can you test it again after updating to latest central?
We landed a fix for that in Bug 1486870, and here's what I get when I copy the message + stacktrace and copy it here:

Error while calling actor 'parentProcessTarget's method 'ensureCSSErrorReportingEnabled' Argument 1 of InspectorUtils.getAllStyleSheets is not an object. protocol.js:1048:5
    writeError resource://devtools/shared/protocol.js:1048
    handler resource://devtools/shared/protocol.js:1243

I'm setting this bug as duplicate, but don't hesitate to reopen it if you still see the issue. Thanks!

Yeah, this seems to be fixed in today's nightly. Thanks!