doc: Add a spec for Application State (#7972)

This commit introduces a specification for "cross-session" app state
that isn't stored in the user's settings.json.

Specs #8324

doc/specs/#8324 - Application State (TSM).md

@@ -0,0 +1,99 @@
+---
+author: Dustin Howett @DHowett
+created on: 2020-10-19
+last updated: 2020-11-18
+issue id: #8324
+---
+
+# Application State (in the Terminal Settings Model)
+
+## Abstract
+
+Terminal is going to need a place to store application "state", including:
+* Dialogs the user has chosen to hide with a `[ ] Do not ask again` checkbox, as proposed in [issue 6641]
+* Which dynamic profiles have been generated, as a way to resolve [user dissatisfaction] around profiles "coming back"
+* On-screen position of the window, active session state, layout, etc. for [eventual restoration]
+
+This specification provides for a place to store these things.
+
+## Inspiration
+
+Many other applications store cross-session state somewhere.
+
+## Solution Design
+
+It is the author's opinion that the above-mentioned settings are inappropriate for storage in the user's settings.json;
+they do not need to be propagated immediately to other instances of Windows Terminal, they are not intended to be
+edited, and storing them outside of settings.json defers the risk inherent in patching the user's settings file.
+
+Therefore, this document proposes that a separate application state archive (`state.json`) be stored next to
+`settings.json`.
+
+It would be accessed by way of an API hosted in the `Microsoft.Terminal.Settings` namespace.
+
+```idl
+namespace Microsoft.Terminal.Settings {
+    [default_interface]
+    runtimeclass ApplicationState {
+        // GetForCurrentApplication will return an object deserialized from state.json.
+        static ApplicationState GetForCurrentApplication();
+
+        void Clear();
+
+        IVector<guid> GeneratedProfiles;
+        Boolean ShowCloseOnExitWarning;
+        // ... further settings ...
+    }
+}
+```
+
+> 📝 The sole motivation for exposing this from Microsoft.Terminal.Settings is to concentrate JSON de-/serialization in one
+place.
+
+Of note: there is no explicit `Save` or `Commit` mechanism. Changes to the application state are committed durably a
+short duration after they're made.
+
+## UI/UX Design
+
+This has no direct impact on the UI/UX; however, we may want to add a button to general settings page titled "reset all
+dialogs" or "don't not ask me again about those things that, some time ago, I told you to not ask me about".
+
+We do not intend this file to be edited by hand, so it does not have to be user-friendly or serialized with indentation.
+
+## Capabilities
+
+### Accessibility
+
+It is not expected that this proposal will impact accessibility.
+
+### Security
+
+It is not expected that this proposal will impact security, as it uses our _existing_ JSON parser in a new place.
+
+### Reliability
+
+[comment]: # Will the proposed change improve reliability? If not, why make the change?
+
+The comment in this section heavily suggests that we should only make changes that improve reliability, not ones that
+maintain it.
+
+### Compatibility
+
+Some users who were expecting profiles to keep coming back after they delete them will need to adjust their behavior.
+
+### Performance, Power, and Efficiency
+
+## Potential Issues
+
+Another file on disk is another file users might edit. We'll need to throw out the entire application state payload if
+it's been damaged, instead of trying to salvage it, so that we more often "do the right thing."
+
+## Future considerations
+
+This will allow us to implement a number of the features mentioned at the head of this document.
+
+## Resources
+
+[issue 6641]: https://github.com/microsoft/terminal/issues/6641
+[user dissatisfaction]: https://github.com/microsoft/terminal/issues/3231
+[eventual restoration]: https://github.com/microsoft/terminal/issues/961