fix the bug for inverted arrow key behavior in left to right vertical text

Hi all
The behavior of arrow key (left and right) is incorrect when typing left to
right vertical text (Mongolian).
If press left key the caret will move to right line, and if press right key
the caret will move to left line.
I have reported this bug already at here
https://www.libreoffice.org/bugzilla/show_bug.cgi?id=82795

I found that the source code contrlling the behavior of arrow keys is in
this file
sw\source\core\uibase\docvw\edtwin.cxx
the function is
void SwEditWin::KeyInput(const KeyEvent &rKEvt)

For fix this issue, I think the code should be change to below

    if( ( bVertText && ( !bTblCrsr || bVertTable ) ) ||
        ( bTblCrsr && bVertTable ) )
    {
bool *bIsTblr*;//how to detect text direction??
        // Attempt to integrate cursor travelling for mongolian layout does
not work.
        // Thus, back to previous mapping of cursor keys to direction keys.
        if( KEY_UP == nKey ) nKey = KEY_LEFT;
        else if( KEY_DOWN == nKey ) nKey = KEY_RIGHT;
 //----
        else if( KEY_LEFT == nKey )
nKey = *bIsTblr *? KEY_UP : KEY_DOWN;
        else if( KEY_RIGHT == nKey )
nKey = *bIsTblr *? KEY_DOWN : KEY_UP;
//----
    }

But I don't know how to detect the text direction (*bIsTblr *) at here. Can
anybody help me?
Best regards