[libhandy/wip/haecker-felix/flap-widget: 1/4] swipe-tracker: Fix coordinate transformation for scrolling

[Alexander Mikhaylenko <alexm src gnome org>]

commit c8a693d28dfab33dfc10cf012b717905a23f93bc
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Wed Nov 25 15:24:26 2020 +0500

    swipe-tracker: Fix coordinate transformation for scrolling
    
    Event coordinates are relative to the GdkWindow the event was on.
    Meanwhile, gtk_widget_translate_coordinates() transforms coordinates from
    a widget to another widget, instead of from a window to widget. This means
    that it's only correct if the widget and window coordinates match, which is
    not always the case and leads to misplaced swipe area in some cases.

 src/hdy-swipe-tracker.c | 48 +++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 43 insertions(+), 5 deletions(-)
---
diff --git a/src/hdy-swipe-tracker.c b/src/hdy-swipe-tracker.c
index 86e7fd7f..be336b19 100644
--- a/src/hdy-swipe-tracker.c
+++ b/src/hdy-swipe-tracker.c
@@ -102,6 +102,45 @@ enum {
 
 static guint signals[SIGNAL_LAST_SIGNAL];
 
+static gboolean
+get_widget_coordinates (HdySwipeTracker *self,
+                        GdkEvent        *event,
+                        gdouble         *x,
+                        gdouble         *y)
+{
+  GdkWindow *window = gdk_event_get_window (event);
+  gdouble tx, ty, out_x = -1, out_y = -1;
+
+  if (!gdk_event_get_coords (event, &tx, &ty))
+    goto out;
+
+  while (window && window != gtk_widget_get_window (GTK_WIDGET (self->swipeable))) {
+    gint window_x, window_y;
+
+    gdk_window_get_position (window, &window_x, &window_y);
+
+    tx += window_x;
+    ty += window_y;
+
+    window = gdk_window_get_parent (window);
+  }
+
+  if (window) {
+    out_x = tx;
+    out_y = ty;
+    goto out;
+  }
+
+out:
+  if (x)
+    *x = out_x;
+
+  if (y)
+    *y = out_y;
+
+  return out_x >= 0 && out_y >= 0;
+}
+
 static void
 reset (HdySwipeTracker *self)
 {
@@ -474,13 +513,12 @@ handle_scroll_event (HdySwipeTracker *self,
 
     if (is_vertical == is_delta_vertical) {
       if (!capture) {
-        GtkWidget *widget = gtk_get_event_widget (event);
         gdouble event_x, event_y;
 
-        gdk_event_get_coords (event, &event_x, &event_y);
-        gtk_widget_translate_coordinates (widget, GTK_WIDGET (self->swipeable),
-                                          event_x, event_y,
-                                          &self->start_x, &self->start_y);
+        get_widget_coordinates (self, event, &event_x, &event_y);
+
+        self->start_x = (gint) round (event_x);
+        self->start_y = (gint) round (event_y);
 
         gesture_prepare (self, delta > 0 ? HDY_NAVIGATION_DIRECTION_FORWARD : HDY_NAVIGATION_DIRECTION_BACK, 
FALSE);
       }