3 files changed, 50 insertions, 14 deletions

diff --git a/src/Selection.cxx b/src/Selection.cxx
index 7b517cc4a..20fb1efca 100644
--- a/src/Selection.cxx
+++ b/src/Selection.cxx
@@ -170,20 +170,15 @@ bool SelectionRange::ContainsCharacter(SelectionPosition spCharacter) const noex
 }
 
 SelectionSegment SelectionRange::Intersect(SelectionSegment check) const noexcept {
-	const SelectionSegment inOrder(caret, anchor);
-	if ((inOrder.start <= check.end) || (inOrder.end >= check.start)) {
-		SelectionSegment portion = check;
-		if (portion.start < inOrder.start)
-			portion.start = inOrder.start;
-		if (portion.end > inOrder.end)
-			portion.end = inOrder.end;
-		if (portion.start > portion.end)
-			return SelectionSegment();
-		else
-			return portion;
-	} else {
-		return SelectionSegment();
-	}
+	const SelectionSegment inOrder = AsSegment();
+	if ((inOrder.start > check.end) || (inOrder.end < check.start)) {
+		// Nothing in common, not even touching so return empty *invalid* segment
+		return {};
+	}
+	return {
+		std::max(check.start, inOrder.start),
+		std::min(check.end, inOrder.end)
+	};
 }
 
 void SelectionRange::Swap() noexcept {
diff --git a/src/Selection.h b/src/Selection.h
index 57de92a57..8ffcbb0bb 100644
--- a/src/Selection.h
+++ b/src/Selection.h
@@ -81,6 +81,12 @@ struct SelectionSegment {
 			end = a;
 		}
 	}
+	constexpr SelectionSegment(Sci::Position a, Sci::Position b) :
+		SelectionSegment(SelectionPosition(a), SelectionPosition(b)) {
+	}
+	[[nodiscard]] constexpr bool operator ==(const SelectionSegment &other) const noexcept {
+		return (start == other.start) && (end == other.end);
+	}
 	bool Empty() const noexcept {
 		return start == end;
 	}
@@ -115,6 +121,9 @@ struct SelectionRange {
 	constexpr SelectionRange(Sci::Position caret_, Sci::Position anchor_) noexcept : caret(caret_), anchor(anchor_) {
 	}
 	explicit SelectionRange(std::string_view sv);
+	SelectionSegment AsSegment() const noexcept {
+		return {caret, anchor};
+	}
 	bool Empty() const noexcept {
 		return anchor == caret;
 	}
diff --git a/test/unit/testSelection.cxx b/test/unit/testSelection.cxx
index 64148bf61..d708607d2 100644
--- a/test/unit/testSelection.cxx
+++ b/test/unit/testSelection.cxx
@@ -162,6 +162,38 @@ TEST_CASE("SelectionRange") {
 		REQUIRE(range123Returned == range123);
 	}
 
+	SECTION("Intersect") {
+		constexpr SelectionSegment segmentEmpty;
+
+		// Range from 1 to 2 with 3 virtual spaces
+		const SelectionRange range123(SelectionPosition(2, 3), SelectionPosition(1));
+		const SelectionSegment segment12(1, 2);
+		const SelectionSegment inside = range123.Intersect(segment12);
+		REQUIRE(inside == segment12);
+
+		const SelectionSegment segment121(SelectionPosition(1), SelectionPosition(2, 1));
+		const SelectionSegment withVirtual = range123.Intersect(segment121);
+		REQUIRE(withVirtual == segment121);
+
+		const SelectionSegment segment052(SelectionPosition(0), SelectionPosition(5, 2));
+		const SelectionSegment internal = range123.Intersect(segment052);	// All inside
+		REQUIRE(internal == range123.AsSegment());
+
+		const SelectionSegment wayOut(SelectionPosition(100), SelectionPosition(105, 2));
+		const SelectionSegment nothing = range123.Intersect(wayOut);
+		REQUIRE(nothing == segmentEmpty);
+
+		const SelectionSegment edge(1, 1);
+		const SelectionSegment nowt = range123.Intersect(edge);
+		REQUIRE(nowt == edge);
+
+		// (0, 1) and (1, 2v3) touch so intersection is a single position.
+		const SelectionSegment front(0, 1);
+		const SelectionSegment single(1, 1);
+		const SelectionSegment thin = range123.Intersect(front);
+		REQUIRE(thin == single);
+	}
+
 }
 
 TEST_CASE("Selection") {