fix clamping the selection at top/bottom properly

src/cascadia/TerminalCore/TerminalSelection.cpp

@@ -385,7 +385,7 @@ void Terminal::UpdateSelection(SelectionDirection direction, SelectionExpansion
     }
     auto targetPos{ WI_IsFlagSet(_selectionEndpoint, SelectionEndpoint::Start) ? _selection->start : _selection->end };
 
-    // 2.A) Perform the movement
+    // 2 Perform the movement
     switch (mode)
     {
     case SelectionExpansion::Char:
@@ -402,9 +402,6 @@ void Terminal::UpdateSelection(SelectionDirection direction, SelectionExpansion
         break;
     }
 
-    // 2.B) Clamp the movement to the mutable viewport
-    targetPos = std::min(targetPos, _GetMutableViewport().BottomRightInclusive());
-
     // 3. Actually modify the selection state
     _selectionMode = std::max(_selectionMode, SelectionInteractionMode::Keyboard);
     if (_selectionMode == SelectionInteractionMode::Mark && !mods.IsShiftPressed())
@@ -471,13 +468,16 @@ void Terminal::_MoveByChar(SelectionDirection direction, til::point& pos)
     case SelectionDirection::Up:
     {
         const auto bufferSize{ _activeBuffer().GetSize() };
-        pos = { pos.X, std::clamp(pos.Y - 1, bufferSize.Top(), bufferSize.BottomInclusive()) };
+        const auto newY{ pos.Y - 1 };
+        pos = newY < bufferSize.Top() ? bufferSize.Origin() : til::point{ pos.X, newY };
         break;
     }
     case SelectionDirection::Down:
     {
         const auto bufferSize{ _activeBuffer().GetSize() };
-        pos = { pos.X, std::clamp(pos.Y + 1, bufferSize.Top(), bufferSize.BottomInclusive()) };
+        const auto mutableBottom{ _GetMutableViewport().BottomInclusive() };
+        const auto newY{ pos.Y + 1 };
+        pos = newY > mutableBottom ? til::point{ bufferSize.RightInclusive(), mutableBottom } : til::point{ pos.X, newY };
         break;
     }
     }