Add support for the DECSWT (Set Window Title) escape sequence (#16804)

This PR adds support for the `OSC 21` sequence used on DEC terminals to
set the window title. It's just an alias of the `OSC 2` title sequence
used by XTerm.

This PR also corrects the handling of blank title sequences, which are
supposed to reset the title to its default value, but were previously
ignored.

## Detailed Description of the Pull Request / Additional comments

To handle the blank title parsing correctly, I had to make some changes
to the state machine. Previously it would not have dispatched an `OSC`
sequence unless it received a semicolon following the `OSC` number, but
when there's a blank string, that semicolon should not be required.

I also took this opportunity to simplify the `OSC` parsing in the state
machine, and eliminate the `_GetOscTitle` method which no longer served
any purpose.

## Validation Steps Performed

I've manually confirmed that the title sequences are now working as
expected, and added some state machine unit tests covering the blank
value handling for these sequences.

I also had to update one of the existing state machine tests to account
for the changes I made to allow the semicolon to be omitted.

Closes #16783
Closes #16784

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

@@ -453,6 +453,7 @@ DECSTBM
 DECSTGLT
 DECSTR
 DECSWL
+DECSWT
 DECTABSR
 DECTCEM
 DECXCPR

src/host/outputStream.cpp

@@ -159,12 +159,13 @@ void ConhostInternalGetSet::WarningBell()
 // Routine Description:
 // - Sets the title of the console window.
 // Arguments:
-// - title - The null-terminated string to set as the window title
+// - title - The string to set as the window title
 // Return Value:
 // - <none>
 void ConhostInternalGetSet::SetWindowTitle(std::wstring_view title)
 {
-    ServiceLocator::LocateGlobals().getConsoleInformation().SetTitle(title);
+    auto& gci = ServiceLocator::LocateGlobals().getConsoleInformation();
+    gci.SetTitle(title.empty() ? gci.GetOriginalTitle() : title);
 }
 
 // Routine Description:

src/terminal/adapter/ITermDispatch.hpp

@@ -63,7 +63,7 @@ class Microsoft::Console::VirtualTerminal::ITermDispatch
     virtual bool ReverseLineFeed() = 0; // RI
     virtual bool BackIndex() = 0; // DECBI
     virtual bool ForwardIndex() = 0; // DECFI
-    virtual bool SetWindowTitle(std::wstring_view title) = 0; // OscWindowTitle
+    virtual bool SetWindowTitle(std::wstring_view title) = 0; // DECSWT, OscWindowTitle
     virtual bool HorizontalTabSet() = 0; // HTS
     virtual bool ForwardTab(const VTInt numTabs) = 0; // CHT, HT
     virtual bool BackwardsTab(const VTInt numTabs) = 0; // CBT

src/terminal/adapter/adaptDispatch.cpp

@@ -2644,7 +2644,7 @@ bool AdaptDispatch::ForwardIndex()
 // Routine Description:
 // - OSC Set Window Title - Sets the title of the window
 // Arguments:
-// - title - The string to set the title to. Must be null terminated.
+// - title - The string to set the title to.
 // Return Value:
 // - True.
 bool AdaptDispatch::SetWindowTitle(std::wstring_view title)

src/terminal/adapter/adaptDispatch.hpp

@@ -100,7 +100,7 @@ namespace Microsoft::Console::VirtualTerminal
         bool ReverseLineFeed() override; // RI
         bool BackIndex() override; // DECBI
         bool ForwardIndex() override; // DECFI
-        bool SetWindowTitle(const std::wstring_view title) override; // OSCWindowTitle
+        bool SetWindowTitle(const std::wstring_view title) override; // DECSWT, OSCWindowTitle
         bool HorizontalTabSet() override; // HTS
         bool ForwardTab(const VTInt numTabs) override; // CHT, HT
         bool BackwardsTab(const VTInt numTabs) override; // CBT

src/terminal/adapter/termDispatch.hpp

@@ -56,7 +56,7 @@ class Microsoft::Console::VirtualTerminal::TermDispatch : public Microsoft::Cons
     bool ReverseLineFeed() override { return false; } // RI
     bool BackIndex() override { return false; } // DECBI
     bool ForwardIndex() override { return false; } // DECFI
-    bool SetWindowTitle(std::wstring_view /*title*/) override { return false; } // OscWindowTitle
+    bool SetWindowTitle(std::wstring_view /*title*/) override { return false; } // DECSWT, OscWindowTitle
     bool HorizontalTabSet() override { return false; } // HTS
     bool ForwardTab(const VTInt /*numTabs*/) override { return false; } // CHT, HT
     bool BackwardsTab(const VTInt /*numTabs*/) override { return false; } // CBT

src/terminal/parser/IStateMachineEngine.hpp

@@ -46,9 +46,7 @@ namespace Microsoft::Console::VirtualTerminal
 
         virtual bool ActionIgnore() = 0;
 
-        virtual bool ActionOscDispatch(const wchar_t wch,
-                                       const size_t parameter,
-                                       const std::wstring_view string) = 0;
+        virtual bool ActionOscDispatch(const size_t parameter, const std::wstring_view string) = 0;
 
         virtual bool ActionSs3Dispatch(const wchar_t wch, const VTParameters parameters) = 0;
 

src/terminal/parser/InputStateMachineEngine.cpp

@@ -538,14 +538,11 @@ bool InputStateMachineEngine::ActionIgnore() noexcept
 // - Triggers the OscDispatch action to indicate that the listener should handle a control sequence.
 //   These sequences perform various API-type commands that can include many parameters.
 // Arguments:
-// - wch - Character to dispatch. This will be a BEL or ST char.
 // - parameter - identifier of the OSC action to perform
 // - string - OSC string we've collected. NOT null terminated.
 // Return Value:
 // - true if we handled the dispatch.
-bool InputStateMachineEngine::ActionOscDispatch(const wchar_t /*wch*/,
-                                                const size_t /*parameter*/,
-                                                const std::wstring_view /*string*/) noexcept
+bool InputStateMachineEngine::ActionOscDispatch(const size_t /*parameter*/, const std::wstring_view /*string*/) noexcept
 {
     return false;
 }

src/terminal/parser/InputStateMachineEngine.hpp

@@ -156,9 +156,7 @@ namespace Microsoft::Console::VirtualTerminal
 
         bool ActionIgnore() noexcept override;
 
-        bool ActionOscDispatch(const wchar_t wch,
-                               const size_t parameter,
-                               const std::wstring_view string) noexcept override;
+        bool ActionOscDispatch(const size_t parameter, const std::wstring_view string) noexcept override;
 
         bool ActionSs3Dispatch(const wchar_t wch, const VTParameters parameters) override;
 

src/terminal/parser/OutputStateMachineEngine.cpp

@@ -766,14 +766,11 @@ bool OutputStateMachineEngine::ActionIgnore() noexcept
 // - Triggers the OscDispatch action to indicate that the listener should handle a control sequence.
 //   These sequences perform various API-type commands that can include many parameters.
 // Arguments:
-// - wch - Character to dispatch. This will be a BEL or ST char.
 // - parameter - identifier of the OSC action to perform
 // - string - OSC string we've collected. NOT null terminated.
 // Return Value:
 // - true if we handled the dispatch.
-bool OutputStateMachineEngine::ActionOscDispatch(const wchar_t /*wch*/,
-                                                 const size_t parameter,
-                                                 const std::wstring_view string)
+bool OutputStateMachineEngine::ActionOscDispatch(const size_t parameter, const std::wstring_view string)
 {
     auto success = false;
 
@@ -782,10 +779,9 @@ bool OutputStateMachineEngine::ActionOscDispatch(const wchar_t /*wch*/,
     case OscActionCodes::SetIconAndWindowTitle:
     case OscActionCodes::SetWindowIcon:
     case OscActionCodes::SetWindowTitle:
+    case OscActionCodes::DECSWT_SetWindowTitle:
     {
-        std::wstring title;
-        success = _GetOscTitle(string, title);
-        success = success && _dispatch->SetWindowTitle(title);
+        success = _dispatch->SetWindowTitle(string);
         break;
     }
     case OscActionCodes::SetColor:
@@ -932,21 +928,6 @@ bool OutputStateMachineEngine::ActionSs3Dispatch(const wchar_t /*wch*/, const VT
     return false;
 }
 
-// Routine Description:
-// - Null terminates, then returns, the string that we've collected as part of the OSC string.
-// Arguments:
-// - string - Osc String input
-// - title - Where to place the Osc String to use as a title.
-// Return Value:
-// - True if there was a title to output. (a title with length=0 is still valid)
-bool OutputStateMachineEngine::_GetOscTitle(const std::wstring_view string,
-                                            std::wstring& title) const
-{
-    title = string;
-
-    return !string.empty();
-}
-
 // Routine Description:
 // - OSC 4 ; c ; spec ST
 //      c: the index of the ansi color table

src/terminal/parser/OutputStateMachineEngine.hpp

@@ -52,9 +52,7 @@ namespace Microsoft::Console::VirtualTerminal
 
         bool ActionIgnore() noexcept override;
 
-        bool ActionOscDispatch(const wchar_t wch,
-                               const size_t parameter,
-                               const std::wstring_view string) override;
+        bool ActionOscDispatch(const size_t parameter, const std::wstring_view string) override;
 
         bool ActionSs3Dispatch(const wchar_t wch, const VTParameters parameters) noexcept override;
 
@@ -217,6 +215,7 @@ namespace Microsoft::Console::VirtualTerminal
             SetForegroundColor = 10,
             SetBackgroundColor = 11,
             SetCursorColor = 12,
+            DECSWT_SetWindowTitle = 21,
             SetClipboard = 52,
             ResetForegroundColor = 110, // Not implemented
             ResetBackgroundColor = 111, // Not implemented
@@ -226,9 +225,6 @@ namespace Microsoft::Console::VirtualTerminal
             ITerm2Action = 1337,
         };
 
-        bool _GetOscTitle(const std::wstring_view string,
-                          std::wstring& title) const;
-
         bool _GetOscSetColorTable(const std::wstring_view string,
                                   std::vector<size_t>& tableIndexes,
                                   std::vector<DWORD>& rgbs) const;

src/terminal/parser/stateMachine.cpp

@@ -685,17 +685,17 @@ void StateMachine::_ActionOscPut(const wchar_t wch)
 }
 
 // Routine Description:
-// - Triggers the CsiDispatch action to indicate that the listener should handle a control sequence.
+// - Triggers the OscDispatch action to indicate that the listener should handle a control sequence.
 //   These sequences perform various API-type commands that can include many parameters.
 // Arguments:
-// - wch - Character to dispatch.
+// - <none>
 // Return Value:
 // - <none>
-void StateMachine::_ActionOscDispatch(const wchar_t wch)
+void StateMachine::_ActionOscDispatch()
 {
     _trace.TraceOnAction(L"OscDispatch");
     _trace.DispatchSequenceTrace(_SafeExecute([=]() {
-        return _engine->ActionOscDispatch(wch, _oscParameter, _oscString);
+        return _engine->ActionOscDispatch(_oscParameter, _oscString);
     }));
 }
 
@@ -1434,20 +1434,28 @@ void StateMachine::_EventCsiSubParam(const wchar_t wch)
 // Routine Description:
 // - Processes a character event into an Action that occurs while in the OscParam state.
 //   Events in this state will:
-//   1. Collect numeric values into an Osc Param
-//   2. Move to the OscString state on a delimiter
-//   3. Ignore everything else.
+//   1. Trigger the OSC action associated with the param on an OscTerminator
+//   2. If we see a ESC, enter the OscTermination state. We'll wait for one
+//      more character before we dispatch the (empty) string.
+//   3. Collect numeric values into an Osc Param
+//   4. Move to the OscString state on a delimiter
+//   5. Ignore everything else.
 // Arguments:
 // - wch - Character that triggered the event
 // Return Value:
 // - <none>
-void StateMachine::_EventOscParam(const wchar_t wch) noexcept
+void StateMachine::_EventOscParam(const wchar_t wch)
 {
     _trace.TraceOnEvent(L"OscParam");
     if (_isOscTerminator(wch))
     {
+        _ActionOscDispatch();
         _EnterGround();
     }
+    else if (_isEscape(wch))
+    {
+        _EnterOscTermination();
+    }
     else if (_isNumericParamValue(wch))
     {
         _ActionOscParam(wch);
@@ -1479,7 +1487,7 @@ void StateMachine::_EventOscString(const wchar_t wch)
     _trace.TraceOnEvent(L"OscString");
     if (_isOscTerminator(wch))
     {
-        _ActionOscDispatch(wch);
+        _ActionOscDispatch();
         _EnterGround();
     }
     else if (_isEscape(wch))
@@ -1511,7 +1519,7 @@ void StateMachine::_EventOscTermination(const wchar_t wch)
     _trace.TraceOnEvent(L"OscTermination");
     if (_isStringTerminatorIndicator(wch))
     {
-        _ActionOscDispatch(wch);
+        _ActionOscDispatch();
         _EnterGround();
     }
     else
@@ -1856,8 +1864,8 @@ void StateMachine::ProcessCharacter(const wchar_t wch)
             ProcessCharacter(_c1To7Bit(wch));
         }
     }
-    // Don't go to escape from the OSC string state - ESC can be used to terminate OSC strings.
-    else if (_isEscape(wch) && _state != VTStates::OscString)
+    // Don't go to escape from the OSC string/param states - ESC can be used to terminate OSC strings.
+    else if (_isEscape(wch) && _state != VTStates::OscString && _state != VTStates::OscParam)
     {
         _ActionInterrupt();
         _EnterEscape();

src/terminal/parser/stateMachine.hpp

@@ -95,7 +95,7 @@ namespace Microsoft::Console::VirtualTerminal
         void _ActionCsiDispatch(const wchar_t wch);
         void _ActionOscParam(const wchar_t wch) noexcept;
         void _ActionOscPut(const wchar_t wch);
-        void _ActionOscDispatch(const wchar_t wch);
+        void _ActionOscDispatch();
         void _ActionSs3Dispatch(const wchar_t wch);
         void _ActionDcsDispatch(const wchar_t wch);
 
@@ -132,7 +132,7 @@ namespace Microsoft::Console::VirtualTerminal
         void _EventCsiIgnore(const wchar_t wch);
         void _EventCsiParam(const wchar_t wch);
         void _EventCsiSubParam(const wchar_t wch);
-        void _EventOscParam(const wchar_t wch) noexcept;
+        void _EventOscParam(const wchar_t wch);
         void _EventOscString(const wchar_t wch);
         void _EventOscTermination(const wchar_t wch);
         void _EventSs3Entry(const wchar_t wch);