Problem with mouse area not reaching lower section

Under Debian 9.3 and 9.8 with OPENBOX and LIGHTDM. I have a problem with the mouse pointer on screen. Depending on the orientation, there is a section on screen that the mouse pointer doesn’t go. BUT, click, move and drag events still occur in this section, just “blindly”. The tested resolution, correctly reported by xrandr, is 1920x1080 or 1080x1920. In “normal” or “left” orientation (--rotate normal, --rotate left), there is approx. 4px on the left of the screen that the pointer doesn’t go into. In “inverted” or “right” orientation, there is a rather large section on the left (approx. 20% of the screen) that the pointer doesn’t go into. BUT, as I said, I can still “move” into this section and click on things. The mouse pointer just stops at the edge of the “invisible line” on screen. If I move in the blind section and “come back”, the pointer “jumps” from where it was stuck to the place where I “surface back” as if the position of the pointer on screen was suddenly updated. I tried playing with --fb and --panning switches to xrandr but it doesn’t correct anything. It extends the desktop and I can pan right or down but the mouse pointer is still sticking to the invisible line. See images : Do anyone have an idea ? Thank you.