[Core] Fix incorrect file sort method

core/string/ustring.cpp

@@ -1043,12 +1043,11 @@ signed char String::naturalnocasecmp_to(const String &p_str) const {
 
 static _FORCE_INLINE_ signed char file_cmp_common(const char32_t *&r_this_str, const char32_t *&r_that_str) {
 	// Compare leading `_` sequences.
-	while (*r_this_str && *r_that_str) {
+	while ((*r_this_str == '_' && *r_that_str) || (*r_this_str && *r_that_str == '_')) {
 		// Sort `_` lower than everything except `.`
-		if (*r_this_str != '_' && *r_that_str == '_') {
+		if (*r_this_str != '_') {
 			return *r_this_str == '.' ? -1 : 1;
-		}
-		if (*r_this_str == '_' && *r_that_str != '_') {
+		} else if (*r_that_str != '_') {
 			return *r_that_str == '.' ? 1 : -1;
 		}
 		r_this_str++;

tests/core/string/test_string.h

@@ -338,10 +338,12 @@ TEST_CASE("[String] Natural compare function test") {
 
 TEST_CASE("[String] File compare function test") {
 	String a = "_img2.png";
+	String b = "img2.png";
 
 	CHECK(a.nocasecmp_to("img10.png") > 0);
 	CHECK_MESSAGE(a.filenocasecmp_to("img10.png") < 0, "Should sort before letters.");
 	CHECK_MESSAGE(a.filenocasecmp_to(".img10.png") > 0, "Should sort after period.");
+	CHECK(b.filenocasecmp_to("img3.png") < 0);
 }
 
 TEST_CASE("[String] hex_encode_buffer") {