Fix: Layouter::GetCharAtPosition counting wrong

Bug introduced in commit 60399e

src/gfx_layout.cpp

@@ -255,9 +255,9 @@ Point Layouter::GetCharPosition(std::string_view::const_iterator ch) const
 }
 
 /**
- * Get the character that is at a position.
+ * Get the character that is at a pixel position in the first line of the layouted text.
  * @param x Position in the string.
- * @return Index of the position or -1 if no character is at the position.
+ * @return String offset of the position (bytes) or -1 if no character is at the position.
  */
 ptrdiff_t Layouter::GetCharAtPosition(int x) const
 {
@@ -278,9 +278,8 @@ ptrdiff_t Layouter::GetCharAtPosition(int x) const
 				size_t index = run.GetGlyphToCharMap()[i];
 
 				size_t cur_idx = 0;
-				int char_index = 0;
-				for (auto str = this->string.begin(); str != this->string.end(); char_index++) {
-					if (cur_idx == index) return char_index;
+				for (auto str = this->string.begin(); str != this->string.end();) {
+					if (cur_idx == index) return str - this->string.begin();
 
 					WChar c = Utf8Consume(str);
 					cur_idx += line->GetInternalCharLength(c);