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- .. _touchpad_jumping_cursor:
- ==============================================================================
- Touchpad jumping cursor bugs
- ==============================================================================
- 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.
- .. note:: This warning is ratelimited and will stop appearing after a few
- times, even if the touchpad jumps continue.
- In most cases, this is a bug in the firmware (or 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 :ref:`libinput record
- <libinput-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.
- If your only encounter with these jumps is the warning printed to the log,
- libinput functions as intended.
- When you encounter the warning in the log, please generate a recording of
- your touchpad with :ref:`libinput record <libinput-record>` and file a bug.
- See :ref:`reporting_bugs` for more details.
- Note that it most cases, libinput cannot actually fix the issue. Filing a
- bug is useful to figure out if there are other factors at play or whether
- there are heuristics we can employ to reduce the impact.
- ------------------------------------------------------------------------------
- AlpsPS/2 ALPS DualPoint TouchPad jumping to 4095/0
- ------------------------------------------------------------------------------
- A special case of pointer jumps happens on ``AlpsPS/2 ALPS DualPoint TouchPad``
- devices found in the Lenovo ThinkPad E465 and E550 and likely others with
- the same touchpad hardware. On those devices, the touchpad occasionally
- sends an event for the second finger to move to position 4095/0 before
- moving back to the original position. libinput detects this movement and
- removes it but depending on the interaction this may cause a smaller jump
- later when the coordinates reset to the new position of the finger.
- Some more information is available in `Gitlab Issue #492 <https://gitlab.freedesktop.org/libinput/libinput/-/issues/492>`__.
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