Refactor Pane to be able to host non-terminal content (#16170)

Instead of `Pane` hosting a `TermControl` directly, it now hosts an
`IPaneContent`. This is an abstraction between the TermControl and the
pane itself, to allow for arbitrary implementations of `IPaneContent`,
with things that might not be terminals.

## References and Relevant Issues

* #997
* #1000

## Detailed Description of the Pull Request / Additional comments

This PR by itself doesn't do much. It's just a refactoring. 
- It doesn't actually add any other types of pane content. 
- It overall just tries to move code whenever possible, with as little
refactoring as possible. There are some patterns from before that don't
super scale well to other types of pane content (think: the `xyzChanged`
events on `IPaneContent`).
- There's a few remaining places where Pane is explicitly checking if
its content is a terminal. We probably shouldn't, but meh

There are two follow-up PRs to this PR:
* #16171 
* #16172 

In addition, there's more work to be done after these merge:
* TODO! issue number for "Replace `IPaneContent::xyzChanged` with
`PropertyChanged` events"
* TODO! issue number for "Re-write state restoration so panes don't
produce `NewTerminalArgs`"

## Validation Steps Performed

* It still launches
* It still works
* Broadcasting still works
* The weird restart connection thing from #16001 still works

## PR Checklist
- [x] Closes #997


## other PRs
* #16170 <-- you are here 
* #16171
* #16172

src/cascadia/LocalTests_TerminalApp/TabTests.cpp

@@ -1326,7 +1326,7 @@ namespace TerminalAppLocalTests
             const auto& controlSettings = activeControl.Settings();
             VERIFY_IS_NOT_NULL(controlSettings);
 
-            VERIFY_ARE_EQUAL(til::color{ 0xff0c0c0c }, controlSettings.DefaultBackground());
+            VERIFY_ARE_EQUAL(til::color{ 0xff0c0c0c }, til::color{ controlSettings.DefaultBackground() });
         });
 
         TestOnUIThread([&page]() {
@@ -1344,7 +1344,7 @@ namespace TerminalAppLocalTests
             VERIFY_IS_NOT_NULL(controlSettings);
 
             Log::Comment(L"Color should be changed to the preview");
-            VERIFY_ARE_EQUAL(til::color{ 0xff000000 }, controlSettings.DefaultBackground());
+            VERIFY_ARE_EQUAL(til::color{ 0xff000000 }, til::color{ controlSettings.DefaultBackground() });
 
             // And we should have stored a function to revert the change.
             VERIFY_ARE_EQUAL(1u, page->_restorePreviewFuncs.size());
@@ -1366,7 +1366,7 @@ namespace TerminalAppLocalTests
             VERIFY_IS_NOT_NULL(controlSettings);
 
             Log::Comment(L"Color should be changed");
-            VERIFY_ARE_EQUAL(til::color{ 0xff000000 }, controlSettings.DefaultBackground());
+            VERIFY_ARE_EQUAL(til::color{ 0xff000000 }, til::color{ controlSettings.DefaultBackground() });
 
             // After preview there should be no more restore functions to execute.
             VERIFY_ARE_EQUAL(0u, page->_restorePreviewFuncs.size());
@@ -1394,7 +1394,7 @@ namespace TerminalAppLocalTests
             const auto& controlSettings = activeControl.Settings();
             VERIFY_IS_NOT_NULL(controlSettings);
 
-            VERIFY_ARE_EQUAL(til::color{ 0xff0c0c0c }, controlSettings.DefaultBackground());
+            VERIFY_ARE_EQUAL(til::color{ 0xff0c0c0c }, til::color{ controlSettings.DefaultBackground() });
         });
 
         TestOnUIThread([&page]() {
@@ -1412,7 +1412,7 @@ namespace TerminalAppLocalTests
             VERIFY_IS_NOT_NULL(controlSettings);
 
             Log::Comment(L"Color should be changed to the preview");
-            VERIFY_ARE_EQUAL(til::color{ 0xff000000 }, controlSettings.DefaultBackground());
+            VERIFY_ARE_EQUAL(til::color{ 0xff000000 }, til::color{ controlSettings.DefaultBackground() });
         });
 
         TestOnUIThread([&page]() {
@@ -1428,7 +1428,7 @@ namespace TerminalAppLocalTests
             VERIFY_IS_NOT_NULL(controlSettings);
 
             Log::Comment(L"Color should be the same as it originally was");
-            VERIFY_ARE_EQUAL(til::color{ 0xff0c0c0c }, controlSettings.DefaultBackground());
+            VERIFY_ARE_EQUAL(til::color{ 0xff0c0c0c }, til::color{ controlSettings.DefaultBackground() });
         });
         Log::Comment(L"Sleep to let events propagate");
         Sleep(250);
@@ -1450,7 +1450,7 @@ namespace TerminalAppLocalTests
             const auto& controlSettings = activeControl.Settings();
             VERIFY_IS_NOT_NULL(controlSettings);
 
-            VERIFY_ARE_EQUAL(til::color{ 0xff0c0c0c }, controlSettings.DefaultBackground());
+            VERIFY_ARE_EQUAL(til::color{ 0xff0c0c0c }, til::color{ controlSettings.DefaultBackground() });
         });
 
         TestOnUIThread([&page]() {
@@ -1467,7 +1467,7 @@ namespace TerminalAppLocalTests
             VERIFY_IS_NOT_NULL(controlSettings);
 
             Log::Comment(L"Color should be changed to the preview");
-            VERIFY_ARE_EQUAL(til::color{ 0xff000000 }, controlSettings.DefaultBackground());
+            VERIFY_ARE_EQUAL(til::color{ 0xff000000 }, til::color{ controlSettings.DefaultBackground() });
         });
 
         TestOnUIThread([&page]() {
@@ -1484,7 +1484,7 @@ namespace TerminalAppLocalTests
             VERIFY_IS_NOT_NULL(controlSettings);
 
             Log::Comment(L"Color should be changed to the preview");
-            VERIFY_ARE_EQUAL(til::color{ 0xffFAFAFA }, controlSettings.DefaultBackground());
+            VERIFY_ARE_EQUAL(til::color{ 0xffFAFAFA }, til::color{ controlSettings.DefaultBackground() });
         });
 
         TestOnUIThread([&page]() {
@@ -1503,7 +1503,7 @@ namespace TerminalAppLocalTests
             VERIFY_IS_NOT_NULL(controlSettings);
 
             Log::Comment(L"Color should be changed");
-            VERIFY_ARE_EQUAL(til::color{ 0xffFAFAFA }, controlSettings.DefaultBackground());
+            VERIFY_ARE_EQUAL(til::color{ 0xffFAFAFA }, til::color{ controlSettings.DefaultBackground() });
         });
         Log::Comment(L"Sleep to let events propagate");
         Sleep(250);

src/cascadia/LocalTests_TerminalApp/pch.h

@@ -56,6 +56,8 @@ Author(s):
 #include <winrt/windows.applicationmodel.core.h>
 
 #include <winrt/Microsoft.Terminal.TerminalConnection.h>
+#include <winrt/Microsoft.Terminal.Core.h>
+#include <winrt/Microsoft.Terminal.Control.h>
 #include <winrt/Microsoft.Terminal.Settings.Model.h>
 
 #include <winrt/Microsoft.UI.Xaml.Controls.h>

src/cascadia/TerminalApp/AppActionHandlers.cpp

@@ -1401,7 +1401,7 @@ namespace winrt::TerminalApp::implementation
         {
             if (const auto activePane{ activeTab->GetActivePane() })
             {
-                _restartPaneConnection(activePane);
+                _restartPaneConnection(activePane->GetContent().try_as<TerminalApp::TerminalPaneContent>(), nullptr);
             }
         }
         args.Handled(true);

src/cascadia/TerminalApp/IPaneContent.idl

@@ -0,0 +1,52 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+namespace TerminalApp
+{
+
+    runtimeclass BellEventArgs
+    {
+        Boolean FlashTaskbar { get; };
+    };
+
+    interface IPaneContent
+    {
+        Windows.UI.Xaml.FrameworkElement GetRoot();
+
+        Windows.Foundation.Size MinimumSize { get; };
+
+        String Title { get; };
+        UInt64 TaskbarState { get; };
+        UInt64 TaskbarProgress { get; };
+        Boolean ReadOnly { get; };
+
+        Microsoft.Terminal.Settings.Model.NewTerminalArgs GetNewTerminalArgs(Boolean asContent);
+
+        void Focus(Windows.UI.Xaml.FocusState reason);
+
+        void Close();
+
+        event Windows.Foundation.TypedEventHandler<IPaneContent, Object> CloseRequested;
+
+        event Windows.Foundation.TypedEventHandler<Object, Object> ConnectionStateChanged;
+        event Windows.Foundation.TypedEventHandler<IPaneContent, BellEventArgs> BellRequested;
+        event Windows.Foundation.TypedEventHandler<IPaneContent, Object> TitleChanged;
+        event Windows.Foundation.TypedEventHandler<IPaneContent, Object> TabColorChanged;
+        event Windows.Foundation.TypedEventHandler<IPaneContent, Object> TaskbarProgressChanged;
+        event Windows.Foundation.TypedEventHandler<IPaneContent, Object> ReadOnlyChanged;
+        event Windows.Foundation.TypedEventHandler<IPaneContent, Object> FocusRequested;
+    };
+
+
+    enum PaneSnapDirection
+    {
+        Width,
+        Height
+    };
+
+    interface ISnappable
+    {
+        Single SnapDownToGrid(PaneSnapDirection direction, Single sizeToSnap);
+        Windows.Foundation.Size GridUnitSize { get; };
+    };
+}