Spend a single shift on the next graphic character, and nothing else

src/XTerm.NET.Tests/VtTestBehaviourTests.cs

@@ -449,4 +449,61 @@ public void DECRC_restores_what_was_designated_not_what_it_resolved_to()
         modeOffAfterSave.Write($"{ShiftOut}@{ShiftIn}");
         Assert.Equal("@", modeOffAfterSave.GetLine(0));       // ASCII, not the French it saved
     }
+
+    /// <summary>
+    /// A single shift is spent by the next graphic character, and by nothing else short of a reset.
+    /// </summary>
+    /// <remarks>
+    /// <para>Four ways in and three answers: SI cancelled a pending shift, SO and the locking
+    /// shifts did not, and RIS could not reach it at all -- the pending state held the TABLE G2
+    /// had resolved to, so a reset put the tables back and the shift went on pointing at the old
+    /// one. ESC * 0, SS2, RIS, 'q' printed a line-drawing dash on a terminal that had just been
+    /// reset to ASCII.</para>
+    ///
+    /// <para>The VT510 manual scopes a single shift to "the next graphic character". A locking
+    /// shift is not one, and neither is a designation -- so a designation between the shift and
+    /// the character it shifts belongs to that character, which is the last case here.</para>
+    /// </remarks>
+    [Fact]
+    public void A_single_shift_is_spent_by_the_next_graphic_character_and_nothing_else()
+    {
+        const string so = "\u000e";
+        const string si = "\u000f";
+
+        // RIS reaches it. Line drawing in G2, shift pending, reset -- the 'q' is a letter again.
+        var reset = Sized(20, 3);
+        reset.Write($"{Esc}*0{Esc}N{Esc}c" + "q");
+        Assert.Equal("q", reset.GetLine(0));
+
+        // The three locking shifts leave it standing, and now agree with each other.
+        foreach (var shift in new[] { si, so, $"{Esc}n" })
+        {
+            var terminal = Sized(20, 3);
+            terminal.Write($"{Esc}*0{Esc}N{shift}q");
+            Assert.Equal("─", terminal.GetLine(0));
+        }
+
+        // A designation after the shift counts: SS2 invokes G2, and what G2 holds is a question
+        // with an answer at print time rather than at shift time.
+        var late = Sized(20, 3);
+        late.Write($"{Esc}N{Esc}*0" + "q");
+        Assert.Equal("─", late.GetLine(0));
+
+        // And it really is spent, on one character and not two.
+        var once = Sized(20, 3);
+        once.Write($"{Esc}*0{Esc}N" + "qq");
+        Assert.Equal("─q", once.GetLine(0));
+
+        // A SUPPLEMENTARY character spends it too. It reaches Print as two UTF-16 code units,
+        // and the clear that spends the shift used to sit inside the single-code-unit branch --
+        // so the shift survived the emoji and landed on whatever came next. That does not skip
+        // the shift, it MOVES it: the q below drew as a box-drawing glyph on a terminal whose G2
+        // the program had finished with, the same class of bug as the RIS case above.
+        //
+        // The emoji itself is untranslated, because a 94-character set has no entry outside the
+        // BMP -- spending the shift and translating through it are different things.
+        var supplementary = Sized(20, 3);
+        supplementary.Write($"{Esc}*0{Esc}N" + "\U0001F600q");
+        Assert.Equal("\U0001F600q", supplementary.GetLine(0));
+    }
 }

src/XTerm.NET/InputHandler.Print.cs

@@ -392,18 +392,32 @@ public void Print(string data)
         var translatedData = data;
         if (data.Length == 1)
         {
-            // A single shift outranks GL for this character and is spent doing it.
-            if (_singleShiftPending)
+            // A single shift outranks GL for this character and is spent doing it. Resolved
+            // here rather than when the shift arrived, so a designation in between counts.
+            if (_singleShift is { } shifted)
             {
-                translatedData = Charsets.TranslateChar(data[0], _singleShiftCharset);
-                _singleShiftCharset = null;
-                _singleShiftPending = false;
+                translatedData = Charsets.TranslateChar(data[0], _charsets.GetValueOrDefault(shifted));
+                _singleShift = null;
             }
             else
             {
                 translatedData = Charsets.TranslateChar(data[0], _activeCharset);
             }
         }
+        else if (_singleShift is not null)
+        {
+            // A supplementary character is a graphic character too, and spends the shift like any
+            // other -- it just has nothing to spend it ON, because a 94- or 96-character set has no
+            // entry outside the BMP and TranslateChar takes a single code unit. Leaving the shift
+            // standing through it does not skip the shift, it MOVES it: the character after the
+            // emoji gets translated instead, so `SS2 <emoji> q` drew q as a box-drawing glyph on a
+            // terminal whose G2 the program had finished with.
+            //
+            // Second, after the length test rather than before it, because that is the branch every
+            // ordinary character takes -- see CLAUDE.md's first section. This one runs only for a
+            // surrogate pair, and only to clear a field.
+            _singleShift = null;
+        }
 
         var width = GetStringCellWidth(translatedData);
 
@@ -812,11 +826,14 @@ public void ShiftOut()
     /// <summary>
     /// Shift In - Select G0 character set (SI, 0x0F).
     /// </summary>
+    /// <remarks>
+    /// A pending single shift survives this, as it survives SO and the locking shifts. SI used
+    /// to cancel one and the other three did not, which is three answers to one question: a
+    /// single shift is spent by the next GRAPHIC character, and a locking shift is not one.
+    /// </remarks>
     public void ShiftIn()
     {
         _currentCharset = CharsetMode.G0;
-        _singleShiftCharset = null;
-        _singleShiftPending = false;
         RefreshActiveCharset();
     }
 
@@ -838,21 +855,28 @@ public void LockingShift(CharsetMode mode)
     /// SS2 (ESC N) and SS3 (ESC O) - invoke G2 or G3 for the NEXT character only.
     /// </summary>
     /// <remarks>
-    /// The single shift is held pending rather than swapped in, so it expires by being consumed
-    /// instead of by something remembering to put the old set back. A shift with no character
-    /// after it simply never fires.
+    /// <para>The single shift is held pending rather than swapped in, so it expires by being
+    /// consumed instead of by something remembering to put the old set back. A shift with no
+    /// character after it simply never fires.</para>
+    ///
+    /// <para>Consumption is the ONLY thing that spends it, short of a reset. The VT510 manual
+    /// says a single shift maps its G-set "for the next graphic character", and neither a
+    /// locking shift nor a designation is one.</para>
     /// </remarks>
-    public void InvokeSingleShift(CharsetMode mode)
-    {
-        _singleShiftCharset = _charsets.GetValueOrDefault(mode);
-        _singleShiftPending = true;
-    }
+    public void InvokeSingleShift(CharsetMode mode) => _singleShift = mode;
 
     /// <summary>
     /// Resets charset state to defaults.
     /// </summary>
     public void ResetCharsets()
     {
+        // A pending single shift is charset state, and RIS is exactly how someone recovers from
+        // a program that died between the shift and the character it was going to shift.
+        //
+        // _charsetIds is no longer cleared here: #149 made the loop below SEED every slot with the
+        // US ASCII designation rather than leave it absent, so clearing first would only empty a
+        // dictionary that is about to have all four entries written.
+        _singleShift = null;
         _ninetySixSets.Clear();
         foreach (var mode in GSets)
         {

src/XTerm.NET/InputHandler.cs

@@ -63,17 +63,21 @@ public partial class InputHandler
     private readonly HashSet<CharsetMode> _ninetySixSets = new();
 
     /// <summary>
-    /// The set a SINGLE shift has invoked for the next printed character, or null.
+    /// The G-set a SINGLE shift has invoked for the next printed character, or null.
     /// </summary>
     /// <remarks>
-    /// Separate from <see cref="_activeCharset"/> because it outranks it for exactly one
+    /// <para>Separate from <see cref="_activeCharset"/> because it outranks it for exactly one
     /// character and then stops: SS2 and SS3 shift the character that follows and nothing
     /// after it. Holding it as pending state rather than swapping the active set is what makes
-    /// "and then stops" automatic instead of something the print path has to remember to undo.
+    /// "and then stops" automatic instead of something the print path has to remember to undo.</para>
+    ///
+    /// <para>The G-SET, not the table it resolved to when the shift arrived. A designation
+    /// between the shift and the character it shifts belongs to that character -- SS2 invokes
+    /// G2, and what G2 holds is a question with an answer at print time. Holding the table also
+    /// meant a reset could not reach it: RIS put the tables back and the pending shift went on
+    /// pointing at the one it had captured.</para>
     /// </remarks>
-    private Dictionary<char, string>? _singleShiftCharset;
-
-    private bool _singleShiftPending;
+    private CharsetMode? _singleShift;
     private CharsetMode _currentCharset;
 
     // Variation selector and combining character constants
@@ -188,7 +192,7 @@ internal void PrintAsciiRun(ReadOnlySpan<byte> data)
         // path can only stop. Rare enough to hand to Print rather than teach twice -- the default
         // is on, so nothing in normal output takes this branch.
         if (!UseRunPrinting || _terminal.InsertMode || _activeCharset is not null
-            || _singleShiftPending
+            || _singleShift is not null
             || _buffer.HasMultiRowSizedRuns || !_terminal.Options.Wraparound)
         {
             foreach (var b in data)
@@ -296,7 +300,7 @@ internal void PrintAsciiRun(string data, int start, int count)
         // path can only stop. Rare enough to hand to Print rather than teach twice -- the default
         // is on, so nothing in normal output takes this branch.
         if (!UseRunPrinting || _terminal.InsertMode || _activeCharset is not null
-            || _singleShiftPending
+            || _singleShift is not null
             || _buffer.HasMultiRowSizedRuns || !_terminal.Options.Wraparound)
         {
             for (var k = 0; k < count; k++)