Fix DECSTR cursor restore in alt screen buffer (#19918)

Author: aarushisingh04

src/host/ut_host/ScreenBufferTests.cpp

@@ -123,6 +123,7 @@ class ScreenBufferTests
     TEST_METHOD(VtResizePreservingAttributes);
 
     TEST_METHOD(VtSoftResetCursorPosition);
+    TEST_METHOD(VtSoftResetAltBufferCursorState);
 
     TEST_METHOD(VtScrollMarginsNewlineColor);
 
@@ -1510,6 +1511,30 @@ void ScreenBufferTests::VtSoftResetCursorPosition()
     VERIFY_ARE_EQUAL(til::point(1, 1), cursor.GetPosition());
 }
 
+void ScreenBufferTests::VtSoftResetAltBufferCursorState()
+{
+    auto& gci = ServiceLocator::LocateGlobals().getConsoleInformation();
+    gci.LockConsole(); // Lock must be taken to manipulate buffer.
+    auto unlock = wil::scope_exit([&] { gci.UnlockConsole(); });
+
+    auto& si = gci.GetActiveOutputBuffer();
+    auto& stateMachine = si.GetStateMachine();
+
+    Log::Comment(L"Move cursor on the main buffer.");
+    stateMachine.ProcessString(L"\x1b[4;7H");
+    VERIFY_ARE_EQUAL(til::point(6, 3), si.GetTextBuffer().GetCursor().GetPosition());
+
+    Log::Comment(L"Enter alt buffer, soft reset, and return to main buffer.");
+    stateMachine.ProcessString(L"\x1b[?1049h");
+    VERIFY_IS_TRUE(gci.GetActiveOutputBuffer()._IsAltBuffer());
+    stateMachine.ProcessString(L"\x1b[!p");
+    stateMachine.ProcessString(L"\x1b[?1049l");
+    VERIFY_IS_FALSE(gci.GetActiveOutputBuffer()._IsAltBuffer());
+
+    Log::Comment(L"Returning from alt buffer should restore the main cursor position.");
+    VERIFY_ARE_EQUAL(til::point(6, 3), gci.GetActiveOutputBuffer().GetTextBuffer().GetCursor().GetPosition());
+}
+
 void ScreenBufferTests::VtScrollMarginsNewlineColor()
 {
     auto& gci = ServiceLocator::LocateGlobals().getConsoleInformation();

src/terminal/adapter/adaptDispatch.cpp

@@ -2981,11 +2981,10 @@ void AdaptDispatch::SoftReset()
     SetGraphicsRendition({}); // Normal rendition.
     SetCharacterProtectionAttribute({}); // Default (unprotected)
 
-    // Reset the saved cursor state.
-    // Note that XTerm only resets the main buffer state, but that
-    // seems likely to be a bug. Most other terminals reset both.
-    _savedCursorState.at(0) = {}; // Main buffer
-    _savedCursorState.at(1) = {}; // Alt buffer
+    // Reset only the active saved cursor state.
+    // This matches xterm behavior when DECSTR is processed while using
+    // the alternate screen buffer (GH#19918).
+    _savedCursorState.at(_usingAltBuffer ? 1 : 0) = {};
 
     // The TerminalOutput state in these buffers must be reset to
     // the same state as the _termOutput instance, which is not