Counters are always generated content and cannot be selected.

third_party/WebKit/Source/core/layout/LayoutCounter.h

Index: third_party/WebKit/Source/core/layout/LayoutCounter.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutCounter.h b/third_party/WebKit/Source/core/layout/LayoutCounter.h
index a5ce9184d05489045bbcfe2227fe4d21a01f8c09..59672bb7a0b58b0a888b7f22c9c6fa35e03000eb 100644
--- a/third_party/WebKit/Source/core/layout/LayoutCounter.h
+++ b/third_party/WebKit/Source/core/layout/LayoutCounter.h
@@ -68,6 +68,7 @@ class LayoutCounter final : public LayoutText {
   void UpdateCounter();
 
   const char* GetName() const override { return "LayoutCounter"; }
+  bool CanBeSelectionLeaf() const final { return false; }
 
  protected:
   void WillBeDestroyed() override;