The touchscreen pointer is pressed while any contact is down and
released when the last one lifts, and the pointer device answers X, Y
and pressed. A multi-touch panel is read by its MT slots only: the
kernel's single-touch emulation (ABS_X/ABS_Y/BTN_TOUCH) repeats the
oldest contact and is ignored there. A single-touch panel keeps its
contact in slot 0, the slot index starts at the kernel's unannounced
slot 0, a slot resync recounts the held contacts, and panel positions
are taken from the axis minimum.
Regression lane: samples/input/udev_touch drives the real driver with
an in-memory multi-touch and single-touch panel.
Closes#20060