diff --git a/src/host/selection.cpp b/src/host/selection.cpp
index 5bc077e9aba..fd815e082ac 100644
--- a/src/host/selection.cpp
+++ b/src/host/selection.cpp
@@ -428,9 +428,9 @@ void Selection::ClearSelection(const bool fStartingNewSelection)
 // - This does not validate whether there is a valid selection right now or not.
 //   It is assumed to already be in a proper selecting state and the given rectangle should be highlighted with the given color unconditionally.
 // Arguments:
-// - psrRect - Rectangular area to fill with color
+// - psrRect - Rectangular area to fill with color (exclusive)
 // - attr - The color attributes to apply
-void Selection::ColorSelection(const til::inclusive_rect& srRect, const TextAttribute attr)
+void Selection::ColorSelection(const til::rect& srRect, const TextAttribute attr)
 {
     auto& gci = ServiceLocator::LocateGlobals().getConsoleInformation();
 
@@ -438,8 +438,8 @@ void Selection::ColorSelection(const til::inclusive_rect& srRect, const TextAttr
     auto& screenInfo = gci.GetActiveOutputBuffer();
 
     til::point coordTargetSize;
-    coordTargetSize.x = CalcWindowSizeX(srRect);
-    coordTargetSize.y = CalcWindowSizeY(srRect);
+    coordTargetSize.x = srRect.width();
+    coordTargetSize.y = srRect.height();
 
     til::point coordTarget;
     coordTarget.x = srRect.left;
@@ -475,9 +475,9 @@ void Selection::ColorSelection(const til::point coordSelectionStart, const til::
         const auto& screenInfo = gci.GetActiveOutputBuffer();
 
         const auto rectangles = screenInfo.GetTextBuffer().GetTextRects(coordSelectionStart, coordSelectionEnd, false, true);
-        for (const auto& rect : rectangles)
+        for (const auto& inclusiveRect : rectangles)
         {
-            ColorSelection(rect, attr);
+            ColorSelection(til::rect{ inclusiveRect }, attr);
         }
     }
     CATCH_LOG();
diff --git a/src/host/selection.hpp b/src/host/selection.hpp
index 8be598651a1..7ff876929ff 100644
--- a/src/host/selection.hpp
+++ b/src/host/selection.hpp
@@ -58,7 +58,7 @@ class Selection
 
     void ClearSelection();
     void ClearSelection(const bool fStartingNewSelection);
-    void ColorSelection(const til::inclusive_rect& srRect, const TextAttribute attr);
+    void ColorSelection(const til::rect& srRect, const TextAttribute attr);
     void ColorSelection(const til::point coordSelectionStart, const til::point coordSelectionEnd, const TextAttribute attr);
 
     // delete these or we can accidentally get copies of the singleton
diff --git a/src/host/selectionInput.cpp b/src/host/selectionInput.cpp
index 9a118ba08ce..d7f08d28bfb 100644
--- a/src/host/selectionInput.cpp
+++ b/src/host/selectionInput.cpp
@@ -698,7 +698,7 @@ bool Selection::_HandleColorSelection(const INPUT_KEY_INFO* const pInputKeyInfo)
             for (auto&& sp : selection)
             {
                 sp.iterate_rows(textBuffer.GetSize().Width(), [&](til::CoordType row, til::CoordType beg, til::CoordType end) {
-                    ColorSelection({ beg, row, end, row }, selectionAttr);
+                    ColorSelection({ beg, row, end, row + 1 }, selectionAttr);
                 });
             }
             ClearSelection();
diff --git a/src/host/selectionState.cpp b/src/host/selectionState.cpp
index 2504800bd93..c74e21dddd5 100644
--- a/src/host/selectionState.cpp
+++ b/src/host/selectionState.cpp
@@ -220,6 +220,23 @@ std::pair<til::point, til::point> Selection::GetSelectionAnchors() const noexcep
     endSelectionAnchor.x = (_d->coordSelectionAnchor.x == _d->srSelectionRect.left) ? _d->srSelectionRect.right : _d->srSelectionRect.left;
     endSelectionAnchor.y = (_d->coordSelectionAnchor.y == _d->srSelectionRect.top) ? _d->srSelectionRect.bottom : _d->srSelectionRect.top;
 
+    // GH #18106: Conhost and Terminal share most of the selection code.
+    //    Both now store the selection data as a half-open range [start, end),
+    //     where "end" is the bottom-right-most point.
+    // Conhost operates as an inclusive range, so we need to adjust the "end" endpoint by incrementing it by one.
+    const auto& gci = ServiceLocator::LocateGlobals().getConsoleInformation();
+    const auto& bufferSize = gci.GetActiveOutputBuffer().GetTextBuffer().GetSize();
+    if (IsLineSelection())
+    {
+        // General comparison for line selection.
+        bufferSize.IncrementInExclusiveBounds(startSelectionAnchor <= endSelectionAnchor ? endSelectionAnchor : startSelectionAnchor);
+    }
+    else
+    {
+        // Compare x-values when we're in block selection!
+        bufferSize.IncrementInExclusiveBounds(startSelectionAnchor.x <= endSelectionAnchor.x ? endSelectionAnchor : startSelectionAnchor);
+    }
+
     if (startSelectionAnchor > endSelectionAnchor)
     {
         return { endSelectionAnchor, startSelectionAnchor };