Spec: Appearance configuration objects for profiles (#8345)

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## Summary of the Pull Request
Spec for #3062

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## PR Checklist
* [ ] Closes #xxx
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] Tests added/passed
* [x] Is documentation
* [ ] Schema updated.
* [x] I work here

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## Detailed Description of the Pull Request / Additional comments
Read the spec

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@@ -22,6 +22,7 @@ Hashtable
 HIGHCONTRASTON
 HIGHCONTRASTW
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+IAppearance
 IAsync
 IBind
 IBox

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 oldnewthing
 opengl
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+pabhojwa
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doc/specs/#3062 - Appearance configuration object for profiles.md

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+---
+author: Pankaj Bhojwani, pabhojwa@microsoft.com
+created on: 2020-11-20
+last updated: 2021-2-5
+issue id: #8345
+---
+
+# Appearance configuration objects for profiles
+
+## Abstract
+
+This spec outlines how we can support 'configuration objects' in our profiles, which
+will allow us to render differently depending on the state of the control. For example, a
+control can be rendered differently if it's focused as compared to when it's unfocused.
+
+## Inspiration
+
+Reference: [#3062](https://github.com/microsoft/terminal/issues/3062)
+
+Users want there to be a more visible indicator than the one we have currently for which
+pane is focused and which panes are unfocused. This change would grant us that feature.
+
+## Solution Design
+
+The implementation design for appearance config objects centers around the recent change where inheritance was added to the
+`TerminalSettings` class in the Terminal Settings Model - i.e. different `TerminalSettings` objects can inherit from each other.
+The reason for this change was that we did not want a settings reload to erase any overrides `TermControl` may have made
+to the settings during runtime. By instead passing a child of the `TerminalSettings` object to the control, we can change
+the parent of the child during a settings reload without the overrides being erased (since those overrides live in the child).
+
+The idea behind unfocused appearance configurations is similar. We will pass in another `TerminalSettings` object to the control,
+which is simply a child that already has some overrides in it. When the control gains or loses focus, it simply switches between
+the two settings objects appropriately.
+
+### Allowed parameters
+
+For now, these states are meant to be entirely appearance-based. So, not all parameters which can be
+defined in a `Profile` can be defined in this new object (for example, we do not want parameters which
+would cause a resize in this object.) Here is the list of parameters we will allow:
+
+- Anything regarding colors: `colorScheme`, `foreground`, `background`, `cursorColor` etc
+- Anything regarding background image: `path`, `opacity`, `alignment`, `stretchMode`
+- `cursorShape`
+
+We may wish to allow further parameters in these objects in the future (like `bellStyle`?). The addition
+of further parameters can be discussed in the future and is out of scope for this spec.
+
+### Inheritance
+
+The inheritance model can be thought of as an 'all-or-nothing' approach in the sense that the `unfocusedAppearance` object
+is considered as a *single* setting instead of an object with many settings. We have chosen this model because it is cleaner
+and easier to understand than the alternative, where each setting within an `unfocusedAppearance` object has a parent from which
+it obtains its value.
+
+Note that when `TerminalApp` initializes a control, it creates a `TerminalSettings` object for that profile and passes the
+control a child of that object (so that the control can store overrides in the child, as described earlier). If an unfocused
+config is defined in the profile (or in globals/profile defaults), then `TerminalApp` will create a *child of that child*,
+put the relevant overrides in it, and pass that into the control as well. Thus, the inheritance of any undefined parameters
+in the unfocused config will be as follows:
+
+1. The unfocused config specified in the profile (or in globals/profile defaults)
+2. Overrides made by the terminal control
+3. The parent profile
+
+## UI/UX Design
+
+Users will be able to add a new setting to their profiles that will look like this:
+
+```
+"unfocusedAppearance":
+{
+    "colorScheme": "Campbell",
+    "cursorColor": "#888",
+    "cursorShape": "emptyBox",
+    "foreground": "#C0C0C0",
+    "background": "#000000"
+}
+```
+
+When certain appearance settings are changed via OSC sequences (such as the background color), only the focused/regular
+appearance will change and the unfocused one will remain unchanged. However, since the unfocused settings object inherits
+from the regular one, it will still apply the change (provided it does not define its own value for that setting).
+
+## Capabilities
+
+### Accessibility
+
+Does not affect accessibility.
+
+### Security
+
+Does not affect security.
+
+### Reliability
+
+This is another location in the settings where parsing/loading the settings may fail. However, this is the case
+for any new setting we add so I would say that this is a reasonable cost for this feature.
+
+### Compatibility
+
+Should not affect compatibility.
+
+### Performance, Power, and Efficiency
+
+Rapidly switching between many panes, causing several successive appearance changes in a short period of time, could
+potentially impact performance. However, regular/reasonable pane switching should not have a noticeable effect.
+
+## Potential Issues
+
+Inactive tabs will be 'rendered' in the background with the `UnfocusedRenderingParams` object, we need to make
+sure that switching to an inactive tab (and so causing the renderer to update with the 'normal' parameters)
+does not cause the window to flash/show a jarring indicator that the rendering values changed.
+
+## Future considerations
+
+We will need to decide how this will look in the settings UI.
+
+We may wish to add more states in the future (like 'elevated'). When that happens, we will need to deal with how
+these appearance objects can scale/layer over each other. We had a lot of discussion about this and could not find
+a suitable solution to the problem of multiple states being valid at the same time (like unfocused and elevated).
+This, along with the fact that it is uncertain if there even will be more states we would want to add led us to
+the conclusion that we should only support the unfocused state for now, and come back to this issue later. If there
+are no more states other than unfocused and elevated, we could allow combining them (like having an 'unfocused elevated' state).
+If there are more states, we could do the implementation as an extension rather than inherently supporting it.
+
+## Resources
+
+