diff --git a/src/prompt_toolkit/completion/base.py b/src/prompt_toolkit/completion/base.py
index 3846ef756..2b59751c8 100644
--- a/src/prompt_toolkit/completion/base.py
+++ b/src/prompt_toolkit/completion/base.py
@@ -422,17 +422,30 @@ def get_suffix(completion: Completion) -> str:
     return _commonprefix([get_suffix(c) for c in completions2])
 
 
+# Data structures that hold coverage information about the conditional branches
+branch_coverage = {
+    "_commonprefix_1": False,  # Branch for checking if not strings
+    "_commonprefix_2": False,  # Branch for comparing characters
+    "_commonprefix_3": False,  # Branch for when the characters do not match
+}
+
 def _commonprefix(strings: Iterable[str]) -> str:
-    # Similar to os.path.commonprefix
     if not strings:
+        branch_coverage["_commonprefix_1"] = True
+        print("Branch 1 was hit")
         return ""
 
     else:
+        branch_coverage["_commonprefix_2"] = True
+        print("Branch 2 was hit")
         s1 = min(strings)
         s2 = max(strings)
 
         for i, c in enumerate(s1):
             if c != s2[i]:
+                branch_coverage["_commonprefix_3"] = True
+                print("Branch 3 was hit")
+                print("here")
                 return s1[:i]
 
         return s1
diff --git a/src/prompt_toolkit/layout/dimension.py b/src/prompt_toolkit/layout/dimension.py
index 2e6f5dd4e..497e6b5d3 100644
--- a/src/prompt_toolkit/layout/dimension.py
+++ b/src/prompt_toolkit/layout/dimension.py
@@ -199,17 +199,30 @@ def to_dimension(value: AnyDimension) -> Dimension:
     raise ValueError("Not an integer or Dimension object.")
 
 
-def is_dimension(value: object) -> TypeGuard[AnyDimension]:
-    """
-    Test whether the given value could be a valid dimension.
-    (For usage in an assertion. It's not guaranteed in case of a callable.)
-    """
+# Initialize coverage tracking global variable
+branch_coverage = {
+    "is_dimension_1": False,  # Branch for checking if value is None
+    "is_dimension_2": False,  # Branch for checking if value is callable
+    "is_dimension_3": False,  # Branch for checking if value is int or Dimension
+    "is_dimension_4": False  # Branch for default case
+}
+
+def is_dimension(value: object) -> bool:
     if value is None:
+        branch_coverage["is_dimension_1"] = True
+        print("Branch 1 was hit")
         return True
     if callable(value):
-        return True  # Assume it's a callable that doesn't take arguments.
+        branch_coverage["is_dimension_2"] = True
+        print("Branch 2 was hit")
+        return True
     if isinstance(value, (int, Dimension)):
+        branch_coverage["is_dimension_3"] = True
+        print("Branch 3 was hit")
         return True
+
+    branch_coverage["is_dimension_4"] = True
+    print("Branch 4 was hit")
     return False
 
 
diff --git a/tests/test_commonprefix_coverage.py b/tests/test_commonprefix_coverage.py
new file mode 100644
index 000000000..72a595f92
--- /dev/null
+++ b/tests/test_commonprefix_coverage.py
@@ -0,0 +1,28 @@
+from __future__ import annotations
+from typing import Iterable, List, Tuple
+from prompt_toolkit.completion.base import branch_coverage,_commonprefix
+
+def print_coverage():
+    print("\nCoverage report:")
+    hit_branches = sum(branch_coverage.values())
+    total_branches = len(branch_coverage)
+    coverage_percentage = (hit_branches / total_branches) * 100
+    for branch, hit in branch_coverage.items():
+        print(f"{branch} was {'hit' if hit else 'not hit'}")
+    print(f"Coverage: {hit_branches}/{total_branches} branches hit ({coverage_percentage:.2f}%)\n")
+
+
+# Tests
+test_cases: List[Tuple[List[str], str]] = [
+    (["car", "car"], "Test 1: Same words"),
+    ([], "Test 2: Empty list"),
+    (["car", "dog"], "Test 2: Different words")
+]
+
+for strings, description in test_cases:
+    print(f"\nTesting case: {description} - Input: {strings}")
+    result = _commonprefix(strings)
+    print("Common prefix:", result)
+
+print_coverage()
+
diff --git a/tests/test_is_dimension_coverage.py b/tests/test_is_dimension_coverage.py
new file mode 100644
index 000000000..4892262b9
--- /dev/null
+++ b/tests/test_is_dimension_coverage.py
@@ -0,0 +1,27 @@
+from prompt_toolkit.layout.dimension import branch_coverage,is_dimension
+
+
+def print_coverage():
+    print("\nCoverage report:")
+    hit_branches = sum(branch_coverage.values())
+    total_branches = len(branch_coverage)
+    coverage_percentage = (hit_branches / total_branches) * 100
+    for branch, hit in branch_coverage.items():
+        print(f"{branch} was {'hit' if hit else 'not hit'}")
+    print(f"Coverage: {hit_branches}/{total_branches} branches hit ({coverage_percentage:.2f}%)\n")
+
+
+# Tests
+test_cases = [
+    (None, "Test 1: None value"),
+    (52, "Test 2: Integer value"),
+    (print_coverage, "Test 3: Regular function"),
+    ("not a dimension", "Test 4: String value (invalid dimension)")
+]
+
+for value, description in test_cases:
+    print(f"\nTesting case: {description}")
+    result = is_dimension(value)
+    print(f"Result: {result}")
+
+print_coverage()
\ No newline at end of file