Fix initial position parsing from -pos tag (#4247)

doc/specs/#1043 - Set the initial position of the Terminal/spec.md

@@ -27,6 +27,7 @@ Two new properties should be added in the json settings file:
 
 2. Both X value and Y values are optional. If any one of them is missing, or the value is invalid, system default value will be used. Examples:
 
+       "1000" equals (1000, 1000)
        ", 1000" equals (default, 1000)
        "1000, " equals (1000, default)
        "," equals (default, default)

src/cascadia/LocalTests_TerminalApp/SettingsTests.cpp

@@ -73,6 +73,8 @@ namespace TerminalAppLocalTests
 
         TEST_METHOD(TestElevateArg);
 
+        TEST_METHOD(TestInitialPositionParsing);
+
         TEST_CLASS_SETUP(ClassSetup)
         {
             return true;
@@ -1619,4 +1621,49 @@ namespace TerminalAppLocalTests
         }
     }
 
-}
+    void SettingsTests::TestInitialPositionParsing()
+    {
+        {
+            const auto pos = LaunchPositionFromString("50");
+            VERIFY_IS_TRUE(pos.X.has_value());
+            VERIFY_IS_TRUE(pos.Y.has_value());
+            VERIFY_ARE_EQUAL(50, pos.X.Value());
+            VERIFY_ARE_EQUAL(50, pos.Y.Value());
+        }
+
+        {
+            const auto pos = LaunchPositionFromString("100,");
+            VERIFY_IS_TRUE(pos.X.has_value());
+            VERIFY_ARE_EQUAL(100, pos.X.Value());
+            VERIFY_IS_FALSE(pos.Y.has_value());
+        }
+
+        {
+            const auto pos = LaunchPositionFromString(",100");
+            VERIFY_IS_FALSE(pos.X.has_value());
+            VERIFY_IS_TRUE(pos.Y.has_value());
+            VERIFY_ARE_EQUAL(100, pos.Y.Value());
+        }
+
+        {
+            const auto pos = LaunchPositionFromString("50,50");
+            VERIFY_IS_TRUE(pos.X.has_value());
+            VERIFY_ARE_EQUAL(50, pos.X.Value());
+            VERIFY_IS_TRUE(pos.Y.has_value());
+            VERIFY_ARE_EQUAL(50, pos.Y.Value());
+        }
+
+        {
+            const auto pos = LaunchPositionFromString("abc,100");
+            VERIFY_IS_FALSE(pos.X.has_value());
+            VERIFY_IS_TRUE(pos.Y.has_value());
+            VERIFY_ARE_EQUAL(100, pos.Y.Value());
+        }
+
+        {
+            const auto pos = LaunchPositionFromString("abc");
+            VERIFY_IS_FALSE(pos.X.has_value());
+            VERIFY_IS_FALSE(pos.Y.has_value());
+        }
+    }
+}

src/cascadia/TerminalSettingsModel/ModelSerializationHelpers.h

@@ -67,6 +67,10 @@ _TIL_INLINEPREFIX ::winrt::Microsoft::Terminal::Settings::Model::LaunchPosition
         string,
         [&initialPosition](int32_t left) { initialPosition.X = left; },
         [&initialPosition](int32_t right) { initialPosition.Y = right; });
+    if (initialPosition.X && !initialPosition.Y && string.find(',') == std::string::npos)
+    {
+        initialPosition.Y = initialPosition.X;
+    }
     return initialPosition;
 }