Thanks for filing the issue! As per issue description looks like issue with fire fox browser.Could you please provide the chrome version,OS details and chrome browser steps of the issue which would help us to triage the issue further.

Thanks in Advance!

The issue isn't reproducible on Chrome.

OS: Reproducible on Linux and Windows 10
Chrome version: Not applicable

Thank you for providing more feedback. Adding the requester to the cc list.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

my guess is this is a bug in Firefox's render path.  can you open the devconsole and see if that text actually exists in the DOM ?

Yes, it's the id="hterm:copy-to-clipboard-source" elements which are becoming visible when selecting several lines of text.

that node is temporary and removed from the DOM as soon as the copy finishes.  are you saying the node remains in the DOM ?

please check the console to see if there are any errors in it.

testing Firefox-52.8.1 (64-bit) locally and it works fine.

That's strange. The node remains in the DOM and a new one is appended every time. I see these errors in the console for each selection:

NS_ERROR_FAILURE:   hterm.js:5297
	hterm.Terminal.prototype.copyStringToClipboard 
	hterm.Terminal.prototype.copySelectionToClipboard 
	hterm.Terminal.prototype.onMouse_ 
	bound

Thrown here:
https://chromium.googlesource.com/apps/libapps/+/master/hterm/js/hterm_terminal.js#3084

Seems it is out of scope from TE end as per C#7 & 8, adding TE-NeedsTraige-help label to move this out of our triaging bucket.

Could someone from dev team please take a look into this issue.
Thanks..!

We keep getting a lot of user reports complaining about this issue.

looks like it's a known bug in firefox:
  https://bugzilla.mozilla.org/show_bug.cgi?id=1178676

i was able to reproduce it on my system running Firefox-60.  posted a workaround:
  https://chromium-review.googlesource.com/1252412

annoyingly, "if (selection.selectAllChildren)" passes, and adding a log says it's a function.  but trying to call it leads to NS_ERROR_FAILURE.