Add Suggestions UI & experimental shell completions support

.github/actions/spelling/allow/allow.txt

@@ -95,6 +95,7 @@ slnt
 Sos
 ssh
 stakeholders
+sxn
 timeline
 timelines
 timestamped

.github/actions/spelling/expect/expect.txt

@@ -315,6 +315,7 @@ CPLINFO
 cplusplus
 CPPCORECHECK
 cppcorecheckrules
+cpprest
 cpprestsdk
 cppwinrt
 CProc
@@ -1452,6 +1453,7 @@ PPEB
 ppf
 ppguid
 ppidl
+pplx
 PPROC
 ppropvar
 ppsi

src/cascadia/LocalTests_TerminalApp/pch.h

@@ -74,3 +74,5 @@ Author(s):
 #include "../../inc/DefaultSettings.h"
 
 #include <cppwinrt_utils.h>
+
+#include <til/winrt.h>

src/cascadia/TerminalApp/CommandPalette.cpp

@@ -225,15 +225,24 @@ namespace winrt::TerminalApp::implementation
     void CommandPalette::_selectedCommandChanged(const IInspectable& /*sender*/,
                                                  const Windows::UI::Xaml::RoutedEventArgs& /*args*/)
     {
+        const auto currentlyVisible{ Visibility() == Visibility::Visible };
+
         const auto selectedCommand = _filteredActionsView().SelectedItem();
         const auto filteredCommand{ selectedCommand.try_as<winrt::TerminalApp::FilteredCommand>() };
         if (_currentMode == CommandPaletteMode::TabSwitchMode)
         {
             _switchToTab(filteredCommand);
         }
-        else if (_currentMode == CommandPaletteMode::ActionMode && filteredCommand != nullptr)
+        else if (_currentMode == CommandPaletteMode::ActionMode &&
+                 currentlyVisible)
         {
-            if (const auto actionPaletteItem{ filteredCommand.Item().try_as<winrt::TerminalApp::ActionPaletteItem>() })
+            // If we don't have a selected command, then end any previews we
+            // might currently be showing.
+            if (filteredCommand == nullptr)
+            {
+                _PreviewActionHandlers(*this, nullptr);
+            }
+            else if (const auto actionPaletteItem{ filteredCommand.Item().try_as<winrt::TerminalApp::ActionPaletteItem>() })
             {
                 _PreviewActionHandlers(*this, actionPaletteItem.Command());
             }
@@ -1083,7 +1092,9 @@ namespace winrt::TerminalApp::implementation
         {
             std::copy(begin(commandsToFilter), end(commandsToFilter), std::back_inserter(actions));
         }
-        else if (_currentMode == CommandPaletteMode::TabSearchMode || _currentMode == CommandPaletteMode::ActionMode || _currentMode == CommandPaletteMode::CommandlineMode)
+        else if (_currentMode == CommandPaletteMode::TabSearchMode ||
+                 _currentMode == CommandPaletteMode::ActionMode ||
+                 _currentMode == CommandPaletteMode::CommandlineMode)
         {
             for (const auto& action : commandsToFilter)
             {