Test: Seamless terminal relaunch

[Tyriar]

Refs: #118066

• macOS @chrmarti
• linux @mjbvz
• windows @devinvalenciano
• web @connor4312

Complexity: 3

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The terminal has a feature that relaunches terminals that have not been interacted with when an extension wants to change the terminal environment, this relaunch will happen automatically already but will cause a flicker, clearing the terminal and launching the new terminal. If the user has interacted with it, there will be a ⚠icon in the top right corner of the terminal.

Seamless terminal relaunch is a new feature that relaunches the terminals (mostly*) without the user even knowing. To test:

• Build and symlink the terminal-sample ext to your insiders extension folder (you might be able to just debug the extension)
• Enable trace logging and open devtools
• Launch a terminal
• Run the Terminal API: Update environment command (or Terminal API: Clear Environment)
• The seamless relaunch should happen ~5+ seconds later, the best way to verify it works is to watch the trace log in the devtools console and look for "relaunch"/"seamless relaunch" trace logs.
• You can also check the environment variable is the correct value (eg. echo $blah, $env:blah), enabling "terminal.integrated.environmentChangesIndicator": "on" will tell you what environment variables should be present

Things to watch out for:

• PowerShell may on launch print how long the profile took to run, it's expected that only this number would update on a seamless relaunch (in a single frame)
• Clicking in the terminal (as well as typing anything) may be considered interacting with the terminal since it can send mouse data to the process
• If you reload the window, the terminal will reconnect to previous processes. These reconnected processes will not work with seamless relaunch.