You can use UI Automation for that, and especially the IUIAutomationTextPattern2 interface that has a GetCaretRange method.

Here are two sample Console app (C++ and C# code) that run continuously and display the caret position for the current element under the mouse:

C++ version

int main()
{
    CoInitializeEx(NULL, COINIT_MULTITHREADED);
    {
        CComPtr<IUIAutomation> automation;

        // make sure you use CLSID_CUIAutomation8, *not* CLSID_CUIAutomation
        automation.CoCreateInstance(CLSID_CUIAutomation8);
        do
        {
            POINT pt;
            if (GetCursorPos(&pt))
            {
                CComPtr<IUIAutomationElement> element;
                automation->ElementFromPoint(pt, &element);
                if (element)
                {
                    CComBSTR name;
                    element->get_CurrentName(&name);
                    wprintf(L"Watched element %s\n", name);

                    CComPtr<IUIAutomationTextPattern2> text;
                    element->GetCurrentPatternAs(UIA_TextPattern2Id, IID_PPV_ARGS(&text));
                    if (text)
                    {
                        // get document range
                        CComPtr<IUIAutomationTextRange> documentRange;
                        text->get_DocumentRange(&documentRange);

                        // get caret range
                        BOOL active = FALSE;
                        CComPtr<IUIAutomationTextRange> range;
                        text->GetCaretRange(&active, &range);
                        if (range)
                        {
                            // compare caret start with document start
                            int caretPos = 0;
                            range->CompareEndpoints(TextPatternRangeEndpoint_Start, documentRange, TextPatternRangeEndpoint_Start, &caretPos);
                            wprintf(L" caret is at %i\n", caretPos);
                        }
                    }
                }
            }
            Sleep(500);
        } while (TRUE);
    }
    CoUninitialize();
    return 0;
}

C# version

static void Main(string[] args)
{
    // needs 'using UIAutomationClient;'
    // to reference UIA, don't use the .NET assembly
    // but instead, reference the UIAutomationClient dll as a COM object
    // and set Embed Interop Types to False for the UIAutomationClient reference in the C# project
    var automation = new CUIAutomation8();
    do
    {
        var cursor = System.Windows.Forms.Cursor.Position;
        var element = automation.ElementFromPoint(new tagPOINT { x = cursor.X, y = cursor.Y });
        if (element != null)
        {
            Console.WriteLine("Watched element " + element.CurrentName);
            var guid = typeof(IUIAutomationTextPattern2).GUID;
            var ptr = element.GetCurrentPatternAs(UIA_PatternIds.UIA_TextPattern2Id, ref guid);
            if (ptr != IntPtr.Zero)
            {
                var pattern = (IUIAutomationTextPattern2)Marshal.GetObjectForIUnknown(ptr);
                if (pattern != null)
                {
                    var documentRange = pattern.DocumentRange;
                    var caretRange = pattern.GetCaretRange(out _);
                    if (caretRange != null)
                    {
                        var caretPos = caretRange.CompareEndpoints(
                            TextPatternRangeEndpoint.TextPatternRangeEndpoint_Start,
                            documentRange,
                            TextPatternRangeEndpoint.TextPatternRangeEndpoint_Start);
                        Console.WriteLine(" caret is at " + caretPos);
                    }
                }
            }
        }
        Thread.Sleep(500);
    }
    while (true);
}

The trick is to use the IUIAutomationTextRange::CompareEndpoints method that allows you to compare the caret range with another range, for example the whole document range.

Note there are drawbacks:

• some apps don't support any MSAA/UIA introspection at all, or don't support the text pattern. For these, there are simply no solution (even using Windows API I think)
• some apps report the caret incorrectly, especially when you select text (so, with a moving caret). For example with Notepad, moving LEFT while pressing SHIFT will select backwards but for some reason UIA doesn't update caret pos. I think it's a problem with Notepad because it also has caret issue with IME (Input Method Editor, like the Emoji editor you can summon using Win+; keyboard combination) which also uses the global caret position. There's no problem with Wordpad for example.