fix(wlr/taskbar): stop forcing tasks onto every output's bar

hide_if_duplicate() unconditionally re-ran handle_output_enter() with
the bar's own wl_output for every non-squashed task, so any app_id or
title event made the task visible on all bars and "all-outputs": false
was effectively ignored. The un-squash path in handle_closed() showed
the replacement task unconditionally, with the same effect.

Track whether the toplevel is actually on the bar's output from the
protocol's output_enter/output_leave events, split the button
show/hide logic out of the protocol handlers, and gate every synthetic
re-show on all-outputs or the tracked output membership.

Fixes #5178

include/modules/wlr/taskbar.hpp

@@ -71,6 +71,10 @@ class Task {
   bool button_visible_ = false;
   bool ignored_ = false;
   bool squashed_ = false;
+  /* Whether the toplevel is on this bar's output, per the protocol's
+   * output_enter/output_leave events */
+  bool on_bar_output_ = false;
+  bool size_allocate_connected_ = false;
 
   bool with_icon_ = false;
   bool with_name_ = false;
@@ -96,6 +100,8 @@ class Task {
   void on_button_size_allocated(Gtk::Allocation& alloc);
   void hide_if_ignored();
   void hide_if_duplicate();
+  void show_button();
+  void hide_button();
 
  public:
   /* Getter functions */