Initialize the VT tab stops when a buffer is created in VT mode

src/host/screenInfo.cpp

@@ -68,6 +68,9 @@ SCREEN_INFORMATION::SCREEN_INFORMATION(
     LineChar[3] = UNICODE_BOX_DRAW_LIGHT_VERTICAL;
     LineChar[4] = UNICODE_BOX_DRAW_LIGHT_UP_AND_RIGHT;
     LineChar[5] = UNICODE_BOX_DRAW_LIGHT_UP_AND_LEFT;
+
+    // Check if VT mode is enabled. Note that this can be true w/o calling
+    // SetConsoleMode, if VirtualTerminalLevel is set to !=0 in the registry.
     const CONSOLE_INFORMATION& gci = ServiceLocator::LocateGlobals().getConsoleInformation();
     if (gci.GetVirtTermLevel() != 0)
     {
@@ -132,6 +135,16 @@ SCREEN_INFORMATION::~SCREEN_INFORMATION()
 
         const NTSTATUS status = pScreen->_InitializeOutputStateMachine();
 
+        if (pScreen->InVTMode())
+        {
+            // microsoft/terminal#411: If VT mode is enabled, lets construct the
+            // VT tab stops. Without this line, if a user has
+            // VirtualTerminalLevel set, then
+            // SetConsoleMode(ENABLE_VIRTUAL_TERMINAL_PROCESSING) won't set our
+            // tab stops, because we're never going from vt off -> on
+            pScreen->SetDefaultVtTabStops();
+        }
+
         if (NT_SUCCESS(status))
         {
             *ppScreen = pScreen;

src/host/ut_host/ScreenBufferTests.cpp

@@ -180,6 +180,8 @@ class ScreenBufferTests
     TEST_METHOD(RestoreDownAltBufferWithTerminalScrolling);
 
     TEST_METHOD(ClearAlternateBuffer);
+
+    TEST_METHOD(InitializeTabStopsInVTMode);
 };
 
 void ScreenBufferTests::SingleAlternateBufferCreationTest()
@@ -4347,3 +4349,25 @@ void ScreenBufferTests::ClearAlternateBuffer()
 
     VerifyText(siMain.GetTextBuffer());
 }
+
+void ScreenBufferTests::InitializeTabStopsInVTMode()
+{
+    // This is a test for microsoft/terminal#411. Refer to that issue for more
+    // context.
+
+    auto& g = ServiceLocator::LocateGlobals();
+    auto& gci = g.getConsoleInformation();
+
+    VERIFY_IS_FALSE(gci.GetActiveOutputBuffer().AreTabsSet());
+
+    // Enable VT mode before we construct the buffer. This emulates setting the
+    // VirtualTerminalLevel reg key before launching the console.
+    gci.SetVirtTermLevel(1);
+
+    // Clean up the old buffer, and re-create it. This new buffer will be
+    // created as if the VT mode was always on.
+    m_state->CleanupGlobalScreenBuffer();
+    m_state->PrepareGlobalScreenBuffer();
+
+    VERIFY_IS_TRUE(gci.GetActiveOutputBuffer().AreTabsSet());
+}