[fix, UX] GestureDetector: fix multiswipe length detection (#4649)

Reported by @poire-z, cf. https://github.com/koreader/koreader/pull/4640#issuecomment-466544922

Apparently it's natural for me to make the second swipe slightly longer than the first, so I never noticed a logic issue. I did notice that it seemed slightly harder to make 4-swipe multiswipes than I expected it to be, but those are not necessarily easy gestures to make.

The problem was that I needed to prevent obviously silly gestures like west west west east. In ignoring such duplication, what I accidentally did was to ignore any further movement west after the first multiswipe direction was detected, meaning that the following swipe east could still end up as a relatively western movement overall.

By simply updating the current multiswipe slot in case of the same direction, both problems are prevented. We'll never get the same direction twice, and X moves over to where it's supposed to be on the left.

frontend/device/gesturedetector.lua

@@ -517,24 +517,23 @@ function GestureDetector:handlePan(tev)
                     ["slot"] = slot,
                 },
             }
-            pan_direction = self:getPath(slot, false, fake_first_tev)
         end
 
-        if msd_cnt == 0
-           or pan_direction ~= msd_direction_prev
-        then
-            local msd_direction, msd_distance = self:getPath(slot, true, fake_first_tev)
+        -- the first time fake_first_tev is nil, so self.first_tevs is automatically used instead
+        local msd_direction, msd_distance = self:getPath(slot, true, fake_first_tev)
 
-            if msd_distance > self.MULTISWIPE_THRESHOLD
-               and (msd_cnt == 0
-                    or not pan_direction:match(msd_direction_prev))
-            then
-                if msd_direction ~= msd_direction_prev then
-                    self.multiswipe_directions[msd_cnt+1] = {
-                        [1] = msd_direction,
-                        [2] = pan_ev,
-                    }
-                end
+        if msd_distance > self.MULTISWIPE_THRESHOLD then
+            if msd_direction ~= msd_direction_prev then
+                self.multiswipe_directions[msd_cnt+1] = {
+                    [1] = msd_direction,
+                    [2] = pan_ev,
+                }
+            -- update ongoing swipe direction to the new maximum
+            else
+                self.multiswipe_directions[msd_cnt] = {
+                    [1] = msd_direction,
+                    [2] = pan_ev,
+                }
             end
         end