Stop committing the whole buffer when determining if it's empty (micr…

…osoft#15582)

As a shortcut, GetLastNonSpaceCharacter can start with the last
committed row. It's guaranteed that there isn't anything of worth below
that point, so why bother checking?

Without this, Terminal immediately commits the entire 9031-line buffer
on startup while trying to--get this!--clear the screen!

---------

Co-authored-by: Leonard Hecker <lhecker@microsoft.com>

src/buffer/out/textBuffer.cpp

@@ -166,6 +166,19 @@ ROW& TextBuffer::_getRowByOffsetDirect(size_t offset)
     return *reinterpret_cast<ROW*>(row);
 }
 
+// Returns the "user-visible" index of the last committed row, which can be used
+// to short-circuit some algorithms that try to scan the entire buffer.
+// Returns 0 if no rows are committed in.
+til::CoordType TextBuffer::_estimateOffsetOfLastCommittedRow() const noexcept
+{
+    const auto lastRowOffset = (_commitWatermark - _buffer.get()) / _bufferRowStride;
+    // This subtracts 2 from the offset to account for the:
+    // * scratchpad row at offset 0, whereas regular rows start at offset 1.
+    // * fact that _commitWatermark points _past_ the last committed row,
+    //   but we want to return an index pointing at the last row.
+    return std::max(0, gsl::narrow_cast<til::CoordType>(lastRowOffset - 2));
+}
+
 // Retrieves a row from the buffer by its offset from the first row of the text buffer
 // (what corresponds to the top row of the screen buffer).
 const ROW& TextBuffer::GetRowByOffset(const til::CoordType index) const
@@ -807,7 +820,7 @@ til::point TextBuffer::GetLastNonSpaceCharacter(std::optional<const Microsoft::C
 
     til::point coordEndOfText;
     // Search the given viewport by starting at the bottom.
-    coordEndOfText.y = viewport.BottomInclusive();
+    coordEndOfText.y = std::min(viewport.BottomInclusive(), _estimateOffsetOfLastCommittedRow());
 
     const auto& currRow = GetRowByOffset(coordEndOfText.y);
     // The X position of the end of the valid text is the Right draw boundary (which is one beyond the final valid character)
@@ -2622,7 +2635,7 @@ HRESULT TextBuffer::Reflow(TextBuffer& oldBuffer,
     // printable char gets reset. See GH #12567
     auto newRowY = newCursor.GetPosition().y + 1;
     const auto newHeight = newBuffer.GetSize().Height();
-    const auto oldHeight = oldBuffer.GetSize().Height();
+    const auto oldHeight = oldBuffer._estimateOffsetOfLastCommittedRow() + 1;
     for (;
          iOldRow < oldHeight && newRowY < newHeight;
          iOldRow++)

src/buffer/out/textBuffer.hpp

@@ -234,6 +234,7 @@ class TextBuffer final
     void _construct(const std::byte* until) noexcept;
     void _destroy() const noexcept;
     ROW& _getRowByOffsetDirect(size_t offset);
+    til::CoordType _estimateOffsetOfLastCommittedRow() const noexcept;
 
     void _SetFirstRowIndex(const til::CoordType FirstRowIndex) noexcept;
     til::point _GetPreviousFromCursor() const;