A common bug encountered on touchpads is a cursor jump when alternating between fingers on a multi-touch-capable touchpad.
For example, after moving the cursor a user may use a second finger in the software button area to physically click the touchpad. Upon setting the finger down, the cursor exhibits a jump towards the bottom left or right, depending on the finger position.
When libinput detects a cursor jump it prints a bug warning to the log with the text "Touch jump detected and discarded." and a link to this page.
In most cases, this is a bug in the kernel driver and to libinput it appears that the touch point moves from its previous position. The pointer jump can usually be seen in the evemu-record output for the device:
E: 249.206319 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- E: 249.218008 0003 0035 3764 # EV_ABS / ABS_MT_POSITION_X 3764 E: 249.218008 0003 0036 2221 # EV_ABS / ABS_MT_POSITION_Y 2221 E: 249.218008 0003 003a 0065 # EV_ABS / ABS_MT_PRESSURE 65 E: 249.218008 0003 0000 3764 # EV_ABS / ABS_X 3764 E: 249.218008 0003 0001 2216 # EV_ABS / ABS_Y 2216 E: 249.218008 0003 0018 0065 # EV_ABS / ABS_PRESSURE 65 E: 249.218008 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- E: 249.230881 0003 0035 3752 # EV_ABS / ABS_MT_POSITION_X 3752 E: 249.230881 0003 003a 0046 # EV_ABS / ABS_MT_PRESSURE 46 E: 249.230881 0003 0000 3758 # EV_ABS / ABS_X 3758 E: 249.230881 0003 0018 0046 # EV_ABS / ABS_PRESSURE 46 E: 249.230881 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- E: 249.242648 0003 0035 1640 # EV_ABS / ABS_MT_POSITION_X 1640 E: 249.242648 0003 0036 4681 # EV_ABS / ABS_MT_POSITION_Y 4681 E: 249.242648 0003 003a 0025 # EV_ABS / ABS_MT_PRESSURE 25 E: 249.242648 0003 0000 1640 # EV_ABS / ABS_X 1640 E: 249.242648 0003 0001 4681 # EV_ABS / ABS_Y 4681 E: 249.242648 0003 0018 0025 # EV_ABS / ABS_PRESSURE 25 E: 249.242648 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- E: 249.254568 0003 0035 1648 # EV_ABS / ABS_MT_POSITION_X 1648 E: 249.254568 0003 003a 0027 # EV_ABS / ABS_MT_PRESSURE 27 E: 249.254568 0003 0000 1644 # EV_ABS / ABS_X 1644 E: 249.254568 0003 0018 0027 # EV_ABS / ABS_PRESSURE 27
In this recording, the pointer jumps from its position 3752/2216 to 1640/4681 within a single frame. On this particular touchpad, this would represent a physical move of almost 50mm. libinput detects some of these jumps and discards the movement but otherwise continues as usual. However, the bug should be fixed at the kernel level.
When you encounter the warning in the log, please generate an evemu recording of your touchpad and file a bug. See Reporting bugs for more details.
