Add a spec for per-profile tab colors

doc/specs/#1337 - Per-Profile Tab Colors/#1337 - Per-Profile Tab Colors.md

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+---
+author: Mike Griese @zadjii-msft
+created on: 2020-07-31
+last updated: 2020-07-31
+issue id: #1337
+---
+# Per-Profile Tab Colors
+
+## Abstract
+
+This spec describes a way to specify tab colors in a profile in a way that will
+be forward compatible with theming the Terminal. This spec will be largely
+dedicated to the design of a single setting, but within the context of theming.
+
+## Inspiration
+
+Following the addition of the Tab Color Picker in [#3789], we've had numerous
+requests for the ability to set the color of a tab directly within a profile.
+While largely we're tracking theming in [#3327], including the specification of
+a tab color, the theming spec ([#5772] )is very large and will take a while to
+revise and approve. This spec is intended to pull a single point out from that
+spec to make it more easily reviewable, and implement it in a way that will
+continue working when we add support for themes in the future.
+
+## Solution Design
+
+To enable per-profile tab colors, we'll add a single setting: `tabColor`. For
+now<sup>[[1](#user-content-footnote-1)]</sup>, this setting will accept any
+`#rrggbb` color string.
+
+Since each profile creates a `Pane` with a `TermControl`, we'll need to store
+this color not in the `Tab`, but somewhere below `Pane`, so that when you switch
+between Panes in a tab with different `tabColor`s, the color will update
+automatically. When a new `TermControl` is created, we'll store this color in the
+`TermControl`'s `Terminal` core. This is to support the future possibility of
+setting the tab color via VT sequences.
+
+A Tab's color will be the result of layering a variety of sources, from the bottom up:
+
+Color | | Set by
+-- | -- | --
+Runtime Color | _optional_ |Color Picker / `setTabColor` action
+Control Tab Color | _optional_ | Profile's `tabColor`, or a color set by VT
+Theme Tab Background | _optional_ | `tab.backgroundColor` in the theme
+Tab Default Color | **default** | TabView in XAML
+
+Some examples:
+* **Scenario 1**: The user has set `"tabColor": "#ff0000"` in their profile.
+  When they create tabs with that profile, instead of appearing in the default
+  color for the TabView, they'll be `#ff0000`.
+* **Scenario 2**: The user has set `"tabColor": "#ff0000"` in their profile.
+  When they try to set the color for that tab (with the color picker) to
+  `#0000ff`, the tab's color is updated to reflect this new blue color. When
+  they clear the runtime color (with the color picker), the tab will return to
+  `#ff0000`.
+* **Scenario 3**: The user has two profiles with colors set, one to `"tabColor":
+  "#ff0000"`, and the other with `"tabColor": "#00ff00"`. If they open these
+  profiles in two panes side-by side, the tab's color will update to reflect the
+  color from the currently-focused control.
+* **Scenario 4**: The user has two profiles with colors set, one to `"tabColor":
+  "#ff0000"`, and the other with `"tabColor": "#00ff00"`. If they open these
+  profiles in two panes side-by side, and try to set the color for that tab
+  (with the color picker) to `#0000ff`, the tab's color is updated to reflect
+  this new blue color. Regardless of which pane is focused, the tab will be
+  blue.
+* **Scenario 5**: The user has set `"tabColor": "#ff0000"` in their profile
+  ("Profile A"), and `"tab.backgroundColor": "#00ff00"`in their theme. When they
+  create tabs with "Profile A", the tabs will appear red. Other tabs (for
+  profiles without `tabColor` set) will appear green, using the color from the
+  theme.
+
+
+## UI/UX Design
+
+In general, this is going to look exactly like the colored tabs look now.
+
+**TODO** Should using the tab color picker to clear a tab's color clear the
+currently set color? E.g if there's a Control Color, but no Runtime Color,
+should clear color remove the Control color to revert to the theme/default
+color?
+
+## Capabilities
+
+<table>
+<tr>
+<td><strong>Accessibility</strong></td>
+<td>
+N/A
+</td>
+</tr>
+<tr>
+<td><strong>Security</strong></td>
+<td>
+N/A
+</td>
+</tr>
+<tr>
+<td><strong>Reliability</strong></td>
+<td>
+No expected change
+</td>
+</tr>
+<tr>
+<td><strong>Compatibility</strong></td>
+<td>
+This entire spec outlines how this feature is designed with a emphasis on future
+compatibility. As such, there are no expected regressions in the future when we
+do add support for themes.
+</td>
+</tr>
+<tr>
+<td><strong>Performance, Power, and Efficiency</strong></td>
+<td>
+No expected change
+</td>
+</tr>
+</table>
+
+## Potential Issues
+
+None expected.
+
+## Footnotes
+
+<a id="footnote-1"><a>[1]: When full theming support is added, themes will
+provide support for setting colors as one of a variety of values:
+
+  * An `#rrggbb` string
+  * The systen accent color
+  * The current background color of the Terminal
+  * A value from a given resource key from XAML
+
+When support for these other types of "smart" colors is added, then the profile
+`tabColor` setting will also gracefully accept these values.
+
+## Future considerations
+
+* It's not out of the realm of possibilty that someone might want to color each
+  _pane_'s color at runtime. In that case, the runtime color would be stored in
+  the `Pane`, not the `Tab`.
+
+
+
+<!-- Footnotes -->
+
+[#1337]: https://github.com/microsoft/terminal/issues/1337
+[#3789]: https://github.com/microsoft/terminal/issues/3789
+[#3327]: https://github.com/microsoft/terminal/issues/3327
+[#5772]: https://github.com/microsoft/terminal/pull/5772