Potential Solution

ImGui.Manual.cs

        public static Vector2 CalcTextSize(string text)
            => CalcTextSizeImpl(text.AsSpan());

        public static Vector2 CalcTextSize(string text, int start)
            => CalcTextSizeImpl(text.AsSpan(start));

        public static Vector2 CalcTextSize(string text, float wrapWidth)
            => CalcTextSizeImpl(text.AsSpan(), wrapWidth: wrapWidth);

        public static Vector2 CalcTextSize(string text, bool hideTextAfterDoubleHash)
            => CalcTextSizeImpl(text.AsSpan(), hideTextAfterDoubleHash: hideTextAfterDoubleHash);

        public static Vector2 CalcTextSize(string text, int start, int length)
            => CalcTextSizeImpl(text.AsSpan(start, length));

        public static Vector2 CalcTextSize(string text, int start, bool hideTextAfterDoubleHash)
            => CalcTextSizeImpl(text.AsSpan(start), hideTextAfterDoubleHash: hideTextAfterDoubleHash);

        public static Vector2 CalcTextSize(string text, int start, float wrapWidth)
            => CalcTextSizeImpl(text.AsSpan(start), wrapWidth: wrapWidth);

        public static Vector2 CalcTextSize(string text, bool hideTextAfterDoubleHash, float wrapWidth)
            => CalcTextSizeImpl(text.AsSpan(), hideTextAfterDoubleHash: hideTextAfterDoubleHash, wrapWidth: wrapWidth);

        public static Vector2 CalcTextSize(string text, int start, int length, bool hideTextAfterDoubleHash)
            => CalcTextSizeImpl(text.AsSpan(start, length), hideTextAfterDoubleHash);

        public static Vector2 CalcTextSize(string text, int start, int length, float wrapWidth)
            => CalcTextSizeImpl(text.AsSpan(start, length), wrapWidth: wrapWidth);

        public static Vector2 CalcTextSize(string text, int start, int length, bool hideTextAfterDoubleHash, float wrapWidth)
            => CalcTextSizeImpl(text.AsSpan(start, length), hideTextAfterDoubleHash, wrapWidth);

        private static Vector2 CalcTextSizeImpl(
            ReadOnlySpan<char> text,
            bool hideTextAfterDoubleHash = false,
            float wrapWidth = -1.0f)
        {
            Vector2 ret;
            byte* nativeTextStart = null;
            byte* nativeTextEnd = null;
            int textByteCount = 0;
            if (text != null)
            {
                textByteCount = Encoding.UTF8.GetByteCount(text);
                if (textByteCount > Util.StackAllocationSizeLimit)
                {
                    nativeTextStart = Util.Allocate(textByteCount + 1);
                }
                else
                {
                    byte* nativeTextStackBytes = stackalloc byte[textByteCount + 1];
                    nativeTextStart = nativeTextStackBytes;
                }

                int nativeTextOffset = Util.GetUtf8(text, nativeTextStart, textByteCount);
                nativeTextStart[nativeTextOffset] = 0;
                nativeTextEnd = nativeTextStart + nativeTextOffset;
            }

            ImGuiNative.igCalcTextSize(&ret, nativeTextStart, nativeTextEnd, *((byte*)(&hideTextAfterDoubleHash)), wrapWidth);
            if (textByteCount > Util.StackAllocationSizeLimit)
            {
                Util.Free(nativeTextStart);
            }

            return ret;
        }