Account for viewport movement when wrapping over multiple rows (#17353)

## Summary of the Pull Request

When we receive a stream of output at the bottom of the page that wraps
over more than two lines, that is expected to pan the viewport down by
multiple rows to accommodate all of the output. However, when the output
is received in a single write, that did not work correctly.

The problem was that we were reusing a `Page` instance across multiple
`_DoLineFeed` calls, and the viewport cached in that `Page` wasn't valid
after the first call. This PR fixes the issue by adjusting the cached
viewport when we determine it has been moved by `_DoLineFeed`.

## References and Relevant Issues

The bug was introduced in PR #16615 when paging support was added.

## Validation Steps Performed

I've verified that the test case in #17351 is now working correctly, and
have added a unit test covering this scenario.

## PR Checklist
- [x] Closes #17351
- [x] Tests added/passed

Author: j4james

src/host/ut_host/ScreenBufferTests.cpp

@@ -262,6 +262,7 @@ class ScreenBufferTests
     TEST_METHOD(CopyDoubleWidthRectangularArea);
 
     TEST_METHOD(DelayedWrapReset);
+    TEST_METHOD(MultilineWrap);
 
     TEST_METHOD(EraseColorMode);
 
@@ -8323,6 +8324,39 @@ void ScreenBufferTests::DelayedWrapReset()
     }
 }
 
+void ScreenBufferTests::MultilineWrap()
+{
+    auto& gci = ServiceLocator::LocateGlobals().getConsoleInformation();
+    auto& si = gci.GetActiveOutputBuffer().GetActiveBuffer();
+    auto& stateMachine = si.GetStateMachine();
+    const auto bufferAttr = si.GetTextBuffer().GetCurrentAttributes();
+    const auto width = si.GetTextBuffer().GetSize().Width();
+    const auto bottomRow = si.GetViewport().BottomInclusive();
+
+    // Starting on the bottom row.
+    si.GetTextBuffer().GetCursor().SetPosition({ 0, bottomRow });
+
+    // Write out enough text to wrap over four lines.
+    auto fourLines = std::wstring{};
+    fourLines += L"1";
+    fourLines += std::wstring(width - 1, L' ');
+    fourLines += L"2";
+    fourLines += std::wstring(width - 1, L' ');
+    fourLines += L"3";
+    fourLines += std::wstring(width - 1, L' ');
+    fourLines += L"4";
+    stateMachine.ProcessString(fourLines);
+
+    Log::Comment(L"Cursor should have moved down three rows");
+    VERIFY_ARE_EQUAL(bottomRow + 3, si.GetTextBuffer().GetCursor().GetPosition().y);
+
+    Log::Comment(L"Bottom four rows should have the content 1, 2, 3, and 4");
+    VERIFY_IS_TRUE(_ValidateLineContains(bottomRow + 0, L"1", bufferAttr));
+    VERIFY_IS_TRUE(_ValidateLineContains(bottomRow + 1, L"2", bufferAttr));
+    VERIFY_IS_TRUE(_ValidateLineContains(bottomRow + 2, L"3", bufferAttr));
+    VERIFY_IS_TRUE(_ValidateLineContains(bottomRow + 3, L"4", bufferAttr));
+}
+
 void ScreenBufferTests::EraseColorMode()
 {
     BEGIN_TEST_METHOD_PROPERTIES()

src/terminal/adapter/PageManager.cpp

@@ -93,6 +93,12 @@ til::CoordType Page::YPanOffset() const noexcept
     return 0; // Vertical panning is not yet supported
 }
 
+void Page::MoveViewportDown() noexcept
+{
+    _viewport.top++;
+    _viewport.bottom++;
+}
+
 PageManager::PageManager(ITerminalApi& api, Renderer& renderer) noexcept :
     _api{ api },
     _renderer{ renderer }

src/terminal/adapter/PageManager.hpp

@@ -33,6 +33,7 @@ namespace Microsoft::Console::VirtualTerminal
         til::CoordType BufferHeight() const noexcept;
         til::CoordType XPanOffset() const noexcept;
         til::CoordType YPanOffset() const noexcept;
+        void MoveViewportDown() noexcept;
 
     private:
         TextBuffer& _buffer;

src/terminal/adapter/adaptDispatch.cpp

@@ -73,14 +73,14 @@ void AdaptDispatch::PrintString(const std::wstring_view string)
 
 void AdaptDispatch::_WriteToBuffer(const std::wstring_view string)
 {
-    const auto page = _pages.ActivePage();
+    auto page = _pages.ActivePage();
     auto& textBuffer = page.Buffer();
     auto& cursor = page.Cursor();
     auto cursorPosition = cursor.GetPosition();
     const auto wrapAtEOL = _api.GetSystemMode(ITerminalApi::Mode::AutoWrap);
     const auto& attributes = page.Attributes();
 
-    const auto [topMargin, bottomMargin] = _GetVerticalMargins(page, true);
+    auto [topMargin, bottomMargin] = _GetVerticalMargins(page, true);
     const auto [leftMargin, rightMargin] = _GetHorizontalMargins(page.Width());
 
     auto lineWidth = textBuffer.GetLineWidth(cursorPosition.y);
@@ -107,7 +107,14 @@ void AdaptDispatch::_WriteToBuffer(const std::wstring_view string)
             // different position from where the EOL was marked.
             if (delayedCursorPosition == cursorPosition)
             {
-                _DoLineFeed(page, true, true);
+                if (_DoLineFeed(page, true, true))
+                {
+                    // If the line feed caused the viewport to move down, we
+                    // need to adjust the page viewport and margins to match.
+                    page.MoveViewportDown();
+                    std::tie(topMargin, bottomMargin) = _GetVerticalMargins(page, true);
+                }
+
                 cursorPosition = cursor.GetPosition();
                 // We need to recalculate the width when moving to a new line.
                 lineWidth = textBuffer.GetLineWidth(cursorPosition.y);
@@ -2505,14 +2512,15 @@ bool AdaptDispatch::CarriageReturn()
 // - withReturn - Set to true if a carriage return should be performed as well.
 // - wrapForced - Set to true is the line feed was the result of the line wrapping.
 // Return Value:
-// - <none>
-void AdaptDispatch::_DoLineFeed(const Page& page, const bool withReturn, const bool wrapForced)
+// - True if the viewport panned down. False if not.
+bool AdaptDispatch::_DoLineFeed(const Page& page, const bool withReturn, const bool wrapForced)
 {
     auto& textBuffer = page.Buffer();
     const auto pageWidth = page.Width();
     const auto bufferHeight = page.BufferHeight();
     const auto [topMargin, bottomMargin] = _GetVerticalMargins(page, true);
     const auto [leftMargin, rightMargin] = _GetHorizontalMargins(pageWidth);
+    auto viewportMoved = false;
 
     auto& cursor = page.Cursor();
     const auto currentPosition = cursor.GetPosition();
@@ -2555,6 +2563,7 @@ void AdaptDispatch::_DoLineFeed(const Page& page, const bool withReturn, const b
         // the end of the buffer.
         _api.SetViewportPosition({ page.XPanOffset(), page.Top() + 1 });
         newPosition.y++;
+        viewportMoved = true;
 
         // And if the bottom margin didn't cover the full page, we copy the
         // lower part of the page down so it remains static. But for a full
@@ -2594,6 +2603,7 @@ void AdaptDispatch::_DoLineFeed(const Page& page, const bool withReturn, const b
 
     cursor.SetPosition(newPosition);
     _ApplyCursorMovementFlags(cursor);
+    return viewportMoved;
 }
 
 // Routine Description:

src/terminal/adapter/adaptDispatch.hpp

@@ -249,7 +249,7 @@ namespace Microsoft::Console::VirtualTerminal
                                              const VTInt rightMargin,
                                              const bool homeCursor = false);
 
-        void _DoLineFeed(const Page& page, const bool withReturn, const bool wrapForced);
+        bool _DoLineFeed(const Page& page, const bool withReturn, const bool wrapForced);
 
         void _DeviceStatusReport(const wchar_t* parameters) const;
         void _CursorPositionReport(const bool extendedReport);