Repository: terminal
Issue: Line breaks are ignored inconsistently when copying text in tmux copy mode
Labels: Product-Conpty,Issue-Bug,Area-TerminalControl,Product-Terminal,Priority-2

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# Environment

Windows build number: `10.0.19042.746`
Windows Terminal version: `1.6.10272.0` (current Preview version from Microsoft Store)

Ubuntu WSL distribution from the Microsoft Store: `2004.2020.812.0`
tmux: `3.0a` (from Ubuntu's APT package repo)

# Steps to reproduce

1\. Open a WSL terminal

2\. Run a new tmux session:

```console
$ tmux
```

3\. Generate some output. _Here a sample output from the command I ran while running into this, for your convenience_

```console
$ cat <<EOF
Install Kubernetes apps from helm charts or YAML files using the "install"
command.

You can also find the post-install message for each app with the "info"
command.

Usage:
  arkade install [flags]
  arkade install [command]

Aliases:
  install, i

Examples:
  arkade install
  arkade install openfaas  --gateways=2
  arkade install inlets-operator --token-file $HOME/do-token
EOF
```



4\. Press `Enter` multiple times, until the output disappears from the current window.

5\. Enter tmux's copy mode with `Ctrl-B [` 

> Note: _first `Ctrl-B`, then `[`. Not simultaneously_

6\. Scroll up with arrow keys or `PgUp`

7\. Copy the previously generated output.

8\. _(optional)_ Press `q` to exit tmux's copy mode.

![image](https://user-images.githubusercontent.com/3299086/106384071-8ec84d00-63c9-11eb-95fe-ba5337ed74e3.png)



# Expected behavior

The copied text contains line breaks at the end of each line.

In this case, I'm expecting:

```
You can also find the post-install message for each app with the "info"
command.

Usage:
  arkade install [flags]
  arkade install [command]

Aliases:
  install, i
```

# Actual behavior

This is the raw pasted text. Notice the spaces before "Aliases" instead of the new line (I can replicate the issue with any text, not only that one):

```
You can also find the post-install message for each app with the "info"
command.

Usage:
  arkade install [flags]
  arkade install [command]
                                                                                                                                                            Aliases:
  install, i
```

ℹ️ **Important note**: this happens only in tmux's copy mode. After pressing `q`, copying the same output will work as expected.