Prevent the virtual viewport bottom being moved up unintentionally

src/host/outputStream.cpp

@@ -685,6 +685,17 @@ bool ConhostInternalGetSet::InsertLines(const size_t count)
     return true;
 }
 
+// Method Description:
+// - Resets the internal "virtual bottom" tracker to the top of the buffer.
+// Arguments:
+// - <none>
+// Return Value:
+// - <none>
+void ConhostInternalGetSet::ResetBottom()
+{
+    _io.GetActiveOutputBuffer().ResetBottom();
+}
+
 // Method Description:
 // - Connects the MoveToBottom call directly into our Driver Message servicing
 //      call inside Conhost.exe

src/host/outputStream.hpp

@@ -126,6 +126,7 @@ class ConhostInternalGetSet final : public Microsoft::Console::VirtualTerminal::
     bool DeleteLines(const size_t count) override;
     bool InsertLines(const size_t count) override;
 
+    void ResetBottom() override;
     bool MoveToBottom() const override;
 
     bool PrivateGetColorTableEntry(const size_t index, COLORREF& value) const noexcept override;

src/host/screenInfo.cpp

@@ -113,14 +113,15 @@ SCREEN_INFORMATION::~SCREEN_INFORMATION()
         // Set up viewport
         pScreen->_viewport = Viewport::FromDimensions({ 0, 0 },
                                                       pScreen->_IsInPtyMode() ? coordScreenBufferSize : coordWindowSize);
-        pScreen->UpdateBottom();
 
         // Set up text buffer
         pScreen->_textBuffer = std::make_unique<TextBuffer>(coordScreenBufferSize,
                                                             defaultAttributes,
                                                             uiCursorSize,
                                                             pScreen->_renderTarget);
 
+        pScreen->UpdateBottom();
+
         const auto& gci = ServiceLocator::LocateGlobals().getConsoleInformation();
         pScreen->_textBuffer->GetCursor().SetColor(gci.GetCursorColor());
         pScreen->_textBuffer->GetCursor().SetType(gci.GetCursorType());
@@ -2520,13 +2521,30 @@ TextBufferCellIterator SCREEN_INFORMATION::GetCellDataAt(const COORD at, const V
 
 // Method Description:
 // - Updates our internal "virtual bottom" tracker with wherever the viewport
-//      currently is.
+//      currently is. This is only used to move the bottom further down, unless
+//      the buffer has shrunk, and the viewport needs to be moved back up to
+//      fit within the new buffer range.
 // - <none>
 // Return Value:
 // - <none>
 void SCREEN_INFORMATION::UpdateBottom()
 {
-    _virtualBottom = _viewport.BottomInclusive();
+    // We clamp it so it's at least as low as the current viewport bottom,
+    // but no lower than the bottom of the buffer.
+    _virtualBottom = std::clamp(_virtualBottom, _viewport.BottomInclusive(), GetBufferSize().BottomInclusive());
+}
+
+// Method Description:
+// - Resets the internal "virtual bottom" tracker to the top of the buffer.
+//      Used when the scrollback buffer has been completely cleared.
+// - <none>
+// Return Value:
+// - <none>
+void SCREEN_INFORMATION::ResetBottom()
+{
+    // The virtual bottom points to the last line of the viewport, so at the
+    // top of the buffer it should be one less than the viewport height.
+    _virtualBottom = _viewport.Height() - 1;
 }
 
 // Method Description:

src/host/screenInfo.hpp

@@ -226,6 +226,7 @@ class SCREEN_INFORMATION : public ConsoleObjectHeader, public Microsoft::Console
     void SetTerminalConnection(_In_ Microsoft::Console::ITerminalOutputConnection* const pTtyConnection);
 
     void UpdateBottom();
+    void ResetBottom();
     void MoveToBottom();
 
     Microsoft::Console::Render::IRenderTarget& GetRenderTarget() noexcept;

src/host/ut_host/ScreenBufferTests.cpp

@@ -67,6 +67,8 @@ class ScreenBufferTests
         auto& currentBuffer = gci.GetActiveOutputBuffer();
         // Make sure a test hasn't left us in the alt buffer on accident
         VERIFY_IS_FALSE(currentBuffer._IsAltBuffer());
+        // Make sure the virtual viewport bottom is reset.
+        currentBuffer._virtualBottom = 0;
         VERIFY_SUCCEEDED(currentBuffer.SetViewportOrigin(true, { 0, 0 }, true));
         // Make sure the viewport always starts off at the default size.
         auto defaultSize = COORD{ CommonState::s_csWindowWidth, CommonState::s_csWindowHeight };
@@ -215,6 +217,7 @@ class ScreenBufferTests
     TEST_METHOD(TestAddHyperlinkCustomIdDifferentUri);
 
     TEST_METHOD(UpdateVirtualBottomWhenCursorMovesBelowIt);
+    TEST_METHOD(DontUpdateVirtualBottomWhenMovedUp);
     TEST_METHOD(RetainHorizontalOffsetWhenMovingToBottom);
 
     TEST_METHOD(TestWriteConsoleVTQuirkMode);
@@ -1542,6 +1545,7 @@ void ScreenBufferTests::VtNewlineOutsideMargins()
     VERIFY_ARE_EQUAL(COORD({ 0, viewportTop + 1 }), si.GetViewport().Origin());
 
     Log::Comment(L"Reset viewport and apply DECSTBM margins");
+    si._virtualBottom = 0;
     VERIFY_SUCCEEDED(si.SetViewportOrigin(true, COORD({ 0, viewportTop }), true));
     stateMachine.ProcessString(L"\x1b[1;5r");
     // Make sure we clear the margins on exit so they can't break other tests.
@@ -3341,6 +3345,7 @@ void ScreenBufferTests::ScrollOperations()
     const auto bufferHeight = si.GetBufferSize().Height();
 
     // Move the viewport down a few lines, and only cover part of the buffer width.
+    si._virtualBottom = 0;
     si.SetViewport(Viewport::FromDimensions({ 5, 10 }, { bufferWidth - 10, 10 }), true);
     const auto viewportStart = si.GetViewport().Top();
     const auto viewportEnd = si.GetViewport().BottomExclusive();
@@ -3865,6 +3870,7 @@ void ScreenBufferTests::EraseTests()
     const auto bufferHeight = si.GetBufferSize().Height();
 
     // Move the viewport down a few lines, and only cover part of the buffer width.
+    si._virtualBottom = 0;
     si.SetViewport(Viewport::FromDimensions({ 5, 10 }, { bufferWidth - 10, 10 }), true);
 
     // Fill the entire buffer with Zs. Blue on Green.
@@ -3985,6 +3991,7 @@ void _CommonScrollingSetup()
     const auto oldView = si.GetViewport();
     const auto view = Viewport::FromDimensions({ 0, 0 }, { oldView.Width(), 6 });
     si.SetViewport(view, true);
+    si.ResetBottom();
     cursor.SetPosition({ 0, 0 });
     stateMachine.ProcessString(L"A");
     cursor.SetPosition({ 0, 5 });
@@ -6021,6 +6028,52 @@ void ScreenBufferTests::UpdateVirtualBottomWhenCursorMovesBelowIt()
     VERIFY_ARE_EQUAL(newVirtualBottom, si.GetViewport().BottomInclusive());
 }
 
+void ScreenBufferTests::DontUpdateVirtualBottomWhenMovedUp()
+{
+    auto& g = ServiceLocator::LocateGlobals();
+    auto& gci = g.getConsoleInformation();
+    auto& si = gci.GetActiveOutputBuffer();
+    auto& cursor = si.GetTextBuffer().GetCursor();
+
+    Log::Comment(L"Set the initial viewport one page from the top");
+    const auto initialOrigin = COORD{ 0, si.GetViewport().Top() + si.GetViewport().Height() };
+    VERIFY_SUCCEEDED(si.SetViewportOrigin(false, initialOrigin, false));
+    VERIFY_ARE_EQUAL(initialOrigin, si.GetViewport().Origin());
+
+    Log::Comment(L"Make sure the initial virtual bottom is at the bottom of the viewport");
+    si.UpdateBottom();
+    const auto initialVirtualBottom = si.GetViewport().BottomInclusive();
+    VERIFY_ARE_EQUAL(initialVirtualBottom, si._virtualBottom);
+
+    Log::Comment(L"Set the initial cursor position 5 lines above virtual bottom line");
+    const auto initialCursorPos = COORD{ 0, initialVirtualBottom - 5 };
+    cursor.SetPosition(initialCursorPos);
+    VERIFY_ARE_EQUAL(initialCursorPos, cursor.GetPosition());
+
+    Log::Comment(L"Pan to the top of the buffer so the the cursor is out of view");
+    const auto topOfBufferOrigin = COORD{ 0, 0 };
+    VERIFY_SUCCEEDED(si.SetViewportOrigin(true, topOfBufferOrigin, false));
+    VERIFY_ARE_EQUAL(topOfBufferOrigin, si.GetViewport().Origin());
+
+    Log::Comment(L"Confirm that the virtual bottom has not changed");
+    VERIFY_ARE_EQUAL(initialVirtualBottom, si._virtualBottom);
+
+    Log::Comment(L"Now write out some content using WriteCharsLegacy");
+    const auto content = L"Hello World";
+    auto numBytes = wcslen(content) * sizeof(wchar_t);
+    VERIFY_SUCCESS_NTSTATUS(WriteCharsLegacy(si, content, content, content, &numBytes, nullptr, 0, 0, nullptr));
+
+    Log::Comment(L"The viewport bottom should now align with the cursor pos");
+    VERIFY_ARE_EQUAL(initialCursorPos.Y, si.GetViewport().BottomInclusive());
+
+    Log::Comment(L"But the virtual bottom should still not have changed");
+    VERIFY_ARE_EQUAL(initialVirtualBottom, si._virtualBottom);
+
+    Log::Comment(L"After MoveToBottom, the viewport should align with the virtual bottom");
+    si.MoveToBottom();
+    VERIFY_ARE_EQUAL(initialVirtualBottom, si.GetViewport().BottomInclusive());
+}
+
 void ScreenBufferTests::RetainHorizontalOffsetWhenMovingToBottom()
 {
     auto& g = ServiceLocator::LocateGlobals();

src/terminal/adapter/adaptDispatch.cpp

@@ -2030,6 +2030,9 @@ bool AdaptDispatch::_EraseScrollback()
 
             if (success)
             {
+                // Reset the virtual viewport bottom to the top of the buffer.
+                _pConApi->ResetBottom();
+
                 // Move the viewport (CAN'T be done in one call with SetConsolescreenBufferInfoEx, because legacy)
                 SMALL_RECT newViewport;
                 newViewport.Left = screen.Left;

src/terminal/adapter/conGetSet.hpp

@@ -90,6 +90,7 @@ namespace Microsoft::Console::VirtualTerminal
         virtual bool DeleteLines(const size_t count) = 0;
         virtual bool InsertLines(const size_t count) = 0;
 
+        virtual void ResetBottom() = 0;
         virtual bool MoveToBottom() const = 0;
 
         virtual bool PrivateGetColorTableEntry(const size_t index, COLORREF& value) const = 0;

src/terminal/adapter/ut_adapter/adapterTest.cpp

@@ -518,6 +518,11 @@ class TestGetSet final : public ConGetSet
         return TRUE;
     }
 
+    void ResetBottom() override
+    {
+        Log::Comment(L"ResetBottom MOCK called...");
+    }
+
     bool MoveToBottom() const override
     {
         Log::Comment(L"MoveToBottom MOCK called...");