Fix: crash when window can't be placed on low resolution screens. (#10932)

Co-authored-by: Jonathan G Rennison <j.g.rennison@gmail.com>

src/core/math_func.hpp

@@ -83,6 +83,32 @@ static inline T Clamp(const T a, const T min, const T max)
 	return a;
 }
 
+/**
+ * Clamp a value between an interval.
+ *
+ * This function returns a value which is between the given interval of
+ * min and max. If the given value is in this interval the value itself
+ * is returned otherwise the border of the interval is returned, according
+ * which side of the interval was 'left'.
+ *
+ * @note If the min value is greater than the max, return value is the average of the min and max.
+ * @param a The value to clamp/truncate.
+ * @param min The minimum of the interval.
+ * @param max the maximum of the interval.
+ * @returns A value between min and max which is closest to a.
+ */
+template <typename T>
+static inline T SoftClamp(const T a, const T min, const T max)
+{
+	if (min > max) {
+		using U = std::make_unsigned_t<T>;
+		return min - (U(min) - max) / 2;
+	}
+	if (a <= min) return min;
+	if (a >= max) return max;
+	return a;
+}
+
 /**
  * Clamp an integer between an interval.
  *

src/misc_gui.cpp

@@ -1140,8 +1140,8 @@ struct QueryWindow : public Window {
 	void FindWindowPlacementAndResize(int def_width, int def_height) override
 	{
 		/* Position query window over the calling window, ensuring it's within screen bounds. */
-		this->left = Clamp(parent->left + (parent->width / 2) - (this->width / 2), 0, _screen.width - this->width);
-		this->top = Clamp(parent->top + (parent->height / 2) - (this->height / 2), 0, _screen.height - this->height);
+		this->left = SoftClamp(parent->left + (parent->width / 2) - (this->width / 2), 0, _screen.width - this->width);
+		this->top = SoftClamp(parent->top + (parent->height / 2) - (this->height / 2), 0, _screen.height - this->height);
 		this->SetDirty();
 	}
 

src/tests/math_func.cpp

@@ -120,3 +120,13 @@ TEST_CASE("ClampTo")
 	/* max uint64_t gets clamped to max int64_t. */
 	CHECK(std::numeric_limits<int64_t>::max() == ClampTo<int64_t>(std::numeric_limits<uint64_t>::max()));
 }
+
+
+TEST_CASE("SoftClamp")
+{
+	/* Special behaviour of soft clamp returning the average of min/max when min is higher than max. */
+	CHECK(1250 == SoftClamp(0, 1500, 1000));
+	int million = 1000 * 1000;
+	CHECK(1250 * million == SoftClamp(0, 1500 * million, 1000 * million));
+	CHECK(0 == SoftClamp(0, 1500 * million, -1500 * million));
+}