Don't push the buffer contents into the scrollback on session restore

[mmseng]

Windows Terminal version

Windows Terminal Preview Version: 1.21.1272.0

Windows build number

10.0.19045.0

Other Software

No response

Summary

I installed Windows Terminal Preview to make use of the new buffer restoration feature, noted as the first bullet here: https://github.com/microsoft/terminal/releases/tag/v1.21.1272.0 (i.e. #16598), but it doesn't seem to behave any differently than previous versions, out of the box. Not sure if the feature is broken, or I'm doing something wrong, or what.

While searching related issues, I saw #17179, but it seems to be related more to special cases while restoring after a reboot due to Windows updates. In my case, I can't get the functionality to work even under the simplest condition of manually closing and reopening Terminal.

Steps to reproduce

1. Install Windows Terminal Preview Version: 1.21.1272.0 from the MS app store.
2. Launch Terminal Preview.
3. Ensure that Settings -> Startup -> When Terminal starts is set to Open windows from a previous session.
4. Run some commands in a tab. In my case I'm just testing with a PowerShell 7 tab with no profile customizations and running Get-Process.
5. Close Terminal Preview.
6. Launch Preview.

Expected Behavior

Original tabs, titles, and contents are restored. In the example case, the single PowerShell 7 tab should have the output of the previously-run Get-Process command displayed in the buffer content, along with however much further buffer history is expected by this feature.

Actual Behavior

Original tabs and tab titles are restored (as in previous versions), but buffer contents are empty (as in previous versions).

Additional info

According to the release notes linked above:

Buffer snapshots are stored in human-readable (albeit UTF-16) VT-encoded text in your package's local state directory, next to your settings.

I do see a file named buffer_<guid>.txt alongside my settings.json and state.json files in the Terminal Preview installation's LocalState folder (where such a file doesn't exist in the production Terminal app's LocalState folder). And that file does contain the output of Get-Process, as expected. But It's apparently just not being pulled into Terminal on a restart. I even see a couple lines in the buffer file that look like [Restored <DateTime>].

[zadjii-msft]

So, this may sound silly, but did you scroll up after the buffer was restored/? The old buffer contents are restored into the scrollback above the new viewport

[mmseng]

Oh lord. You're right. It's there and I just never scrolled up 🤦

I suppose I was expecting the window to look exactly the same as the way I left it (i.e. at the same scrolling position).

Well I'm certainly happy that it works (been desperately wanting this feature for a long time). I guess my feedback is that I would prefer an option to additionally restore the scrolling position. However I recognize that that doesn't necessarily make sense (at least in the current implementation), because the restored buffer is not actually 1:1 with the buffer from before the restoration due to the injection of the [Restored <DateTime>] events, which are actually a nice feature IMO, as they clearly log when the console was restarted.

Thanks for your response.

[lhecker]

You're not the first person to bring this flaw up. We've talked about fixing the issue before, and I'm mostly leaning towards doing so now. However, this will increase the startup cost quite a bit, as the cursor position after session restoration needs to be synced up, etc.

[mmseng]

I do think if that "fix" is added, it would be best as an optional setting (maybe even non-default), as I can definitely see people (perhaps even myself at times) preferring the current behavior. I'm guessing that would also mean people could opt into/out of the additional startup cost, maybe. Just my 2 cents.