ui/gtk: Fix mouse/motion event scaling issue with GTK display backend

Remove gtk_widget_get_scale_factor() usage from the calculation of
the motion events in the GTK backend to make it work correctly on
environments that have `gtk_widget_get_scale_factor() != 1`.

This scale factor usage had been introduced in the commit f14aab420c and
at that time the window size was used for calculating the things and it
was working correctly. However, in the commit 2f31663ed4 the logic
switched to use the widget size instead of window size and because of
the change the usage of scale factor becomes invalid (since widgets use
`vc->gfx.scale_{x, y}` for scaling).

Tested on Crostini on ChromeOS (15823.51.0) with an external display.

Fixes: 2f31663ed4 ("ui/gtk: use widget size for cursor motion event")
Fixes: f14aab420c ("ui: fix incorrect pointer position on highdpi with
gtk")

Signed-off-by: hikalium <hikalium@hikalium.com>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20240512111435.30121-3-hikalium@hikalium.com>

ui/gtk.c

@@ -891,7 +891,7 @@ static gboolean gd_motion_event(GtkWidget *widget, GdkEventMotion *motion,
     int x, y;
     int mx, my;
     int fbh, fbw;
-    int ww, wh, ws;
+    int ww, wh;
 
     if (!vc->gfx.ds) {
         return TRUE;
@@ -899,11 +899,15 @@ static gboolean gd_motion_event(GtkWidget *widget, GdkEventMotion *motion,
 
     fbw = surface_width(vc->gfx.ds) * vc->gfx.scale_x;
     fbh = surface_height(vc->gfx.ds) * vc->gfx.scale_y;
-
     ww = gtk_widget_get_allocated_width(widget);
     wh = gtk_widget_get_allocated_height(widget);
-    ws = gtk_widget_get_scale_factor(widget);
 
+    /*
+     * `widget` may not have the same size with the frame buffer.
+     * In such cases, some paddings are needed around the `vc`.
+     * To achieve that, `vc` will be displayed at (mx, my)
+     * so that it is displayed at the center of the widget.
+     */
     mx = my = 0;
     if (ww > fbw) {
         mx = (ww - fbw) / 2;
@@ -912,8 +916,12 @@ static gboolean gd_motion_event(GtkWidget *widget, GdkEventMotion *motion,
         my = (wh - fbh) / 2;
     }
 
-    x = (motion->x - mx) / vc->gfx.scale_x * ws;
-    y = (motion->y - my) / vc->gfx.scale_y * ws;
+    /*
+     * `motion` is reported in `widget` coordinates
+     * so translating it to the coordinates in `vc`.
+     */
+    x = (motion->x - mx) / vc->gfx.scale_x;
+    y = (motion->y - my) / vc->gfx.scale_y;
 
     trace_gd_motion_event(ww, wh, gtk_widget_get_scale_factor(widget), x, y);