popover: fix focus when inside an unfocused window

Fix popovers to properly gain focus when clicked
inside an unfocused window.

We use the GTK_PHASE_CAPTURE of the 'pressed' event
to early detect that the popover is being clicked
inside an inactive window, this allow us to present
the window (and be focused) before the normal signal
handlers for the popover click/pressed events are run
which would ultimately give focus to popover widget.

This fix works for both modal and 'non modal' popovers,
although for GTK4 we only benefit from the 'non modal'
case, as GTK4 (contrary to GTK3) hides the modal popovers
when the focus is changed to another application's window,
so it's only possible for a 'non modal' popover to be
clicked inside an unfocused window.

Fixes issue #1871

gtk/gtkpopover.c

@@ -110,6 +110,7 @@
 #include "gtknative.h"
 #include "gtkwidgetprivate.h"
 #include "gtkeventcontrollerkey.h"
+#include "gtkgestureclick.h"
 #include "gtkcssnodeprivate.h"
 #include "gtkbindings.h"
 #include "gtkenums.h"
@@ -544,6 +545,20 @@ node_style_changed_cb (GtkCssNode        *node,
     gtk_widget_queue_draw (widget);
 }
 
+static void
+gtk_popover_click_gesture_pressed (GtkGestureClick *gesture,
+                                   gint             n_press,
+                                   gdouble          widget_x,
+                                   gdouble          widget_y,
+                                   GtkPopover      *popover)
+{
+  GtkPopoverPrivate *priv = gtk_popover_get_instance_private (popover);
+  GtkWindow *rel_window = GTK_WINDOW (gtk_widget_get_root (priv->relative_to));
+
+  if (!gtk_window_is_active (rel_window) && gtk_widget_is_drawable (GTK_WIDGET (popover)))
+    gtk_window_present_with_time (rel_window, gtk_get_current_event_time ());
+}
+
 static void
 gtk_popover_init (GtkPopover *popover)
 {
@@ -551,6 +566,7 @@ gtk_popover_init (GtkPopover *popover)
   GtkWidget *widget = GTK_WIDGET (popover);
   GtkEventController *controller;
   GtkStyleContext *context;
+  GtkGesture *click_gesture;
 
   priv->position = GTK_POS_BOTTOM;
   priv->final_position = GTK_POS_BOTTOM;
@@ -572,6 +588,15 @@ gtk_popover_init (GtkPopover *popover)
                            G_CALLBACK (node_style_changed_cb), popover, 0);
   g_object_unref (priv->arrow_node);
 
+  click_gesture = gtk_gesture_click_new ();
+  gtk_gesture_single_set_button (GTK_GESTURE_SINGLE (click_gesture), 0);
+  gtk_gesture_single_set_exclusive (GTK_GESTURE_SINGLE (click_gesture), TRUE);
+  gtk_event_controller_set_propagation_phase (GTK_EVENT_CONTROLLER (click_gesture),
+                                              GTK_PHASE_CAPTURE);
+  g_signal_connect (click_gesture, "pressed",
+                    G_CALLBACK (gtk_popover_click_gesture_pressed), popover);
+  gtk_widget_add_controller (widget, GTK_EVENT_CONTROLLER (click_gesture));
+
   priv->contents_widget = gtk_gizmo_new ("contents",
                                          measure_contents,
                                          allocate_contents,