On clickpads with two touchpoints, try to detect clickfinger 3 actions

Some touchpads provide BTN_TOOL_TRIPLETAP but do not track more than two
touchpoints. For those, try to detect clickfinger3 actions as best as we can.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 9388b54a109bbd634e05611e6c90110d698f13d1)

src/synaptics.c

@@ -2507,6 +2507,28 @@ clickpad_guess_clickfingers(SynapticsPrivate * priv,
         close_point >>= 1;
     }
 
+    /* Some trackpads touchpad only track two touchpoints but announce
+       BTN_TOOL_TRIPLETAP (which sets hw->numFingers to 3).
+       This can affect clickfingers, in the following ways:
+       * one finger down: normal click
+       * two fingers down, close together: 2 finger click
+       * two fingers down, apart: normal click
+       * three fingers down, close together: 3 finger click
+       * three fingers down, with two grouped next to each other: should be
+       * 2-finger click but we can't detect this.
+       * so: if two detected fingers are close together and HW says three
+       * fingers, make it three fingers.
+       * if two detected fingers are apart and HW says three fingers, make
+       * it a two-finger click, guessing that the third finger is somewhere
+       * close to another finger.
+       */
+    if (hw->numFingers >= 3 && nfingers < hw->numFingers) {
+        if (!nfingers) /* touchpoints too far apart */
+            nfingers = 2;
+        else
+            nfingers++;
+    }
+
     return nfingers;
 }