Use NFC form when possible. Fixes issue 2813

ScreenChar.h

@@ -220,8 +220,10 @@ UTF32Char CharToLongChar(unichar code, BOOL isComplex);
 // returned.
 int AppendToComplexChar(int key, unichar codePoint);
-// Create a new complex char from two code points. A key is returned.
-int BeginComplexChar(unichar initialCodePoint, unichar combiningChar);
+// Takes a non-complex character and adds a combining mark to it. It may or may not
+// become complex as a result, depending on whether there is an NFC form for the
+// new composite.
+void BeginComplexChar(screen_char_t *screenChar, unichar combiningChar);
 // Create or lookup & return the code for a complex char.
 int GetOrSetComplexChar(NSString* str);

ScreenChar.m

@@ -149,16 +149,25 @@ int AppendToComplexChar(int key, unichar codePoint)
     return GetOrSetComplexChar(temp);
 }
-int BeginComplexChar(unichar initialCodePoint, unichar combiningChar)
+void BeginComplexChar(screen_char_t *screenChar, unichar combiningChar)
 {
+    unichar initialCodePoint = screenChar->code;
     if (initialCodePoint == UNICODE_REPLACEMENT_CHAR) {
-        return UNICODE_REPLACEMENT_CHAR;
+        return;
     }
     unichar temp[2];
     temp[0] = initialCodePoint;
     temp[1] = combiningChar;
-    return GetOrSetComplexChar([NSString stringWithCharacters:temp length:2]);
+    
+    // See if it makes a single code in NFC.
+    NSString *nfc = [[NSString stringWithCharacters:temp length:2] precomposedStringWithCanonicalMapping];
+    if (nfc.length == 1) {
+        screenChar->code = [nfc characterAtIndex:0];
+    } else {
+        screenChar->code = GetOrSetComplexChar([NSString stringWithCharacters:temp length:2]);
+        screenChar->complexChar = YES;
+    }
 }
 BOOL StringContainsCombiningMark(NSString *s)

VT100Screen.m

@@ -205,8 +205,7 @@ void StringToScreenChars(NSString *s,
                     // built by surrogates.
                     buf[j].code = AppendToComplexChar(buf[j].code, sc[i]);
                 } else {
-                    buf[j].code = BeginComplexChar(buf[j].code, sc[i]);
-                    buf[j].complexChar = YES;
+                    BeginComplexChar(buf + j, sc[i]);
                 }
                 if (IsLowSurrogate(sc[i])) {
                     NSString* str = ComplexCharToStr(buf[j].code);
@@ -2702,9 +2701,7 @@ void DumpBuf(screen_char_t* p, int n) {
         theLine[cx].code = AppendToComplexChar(theLine[cx].code,
                                                combiningChar);
     } else {
-        theLine[cx].code = BeginComplexChar(theLine[cx].code,
-                                            combiningChar);
-        theLine[cx].complexChar = YES;
+        BeginComplexChar(theLine + cx, combiningChar);
     }
     return YES;
 }