fix: the sidebar button follows the panel, and the wish survives having no source

The toolbar button was the panel's only input: `update_side_visibility()` derived the
panel from the button and never the other way, so the button went on saying "shown" in
two cases where the panel was off -- while there were no signals to show, and after
anything else moved the split view's state.

Binding the two together is not the fix. AdwOverlaySplitView writes `show-sidebar`
itself, at startup and when it collapses, with no user involved; a binding pushes that
into the button, the button's handler reads it as the user's wish, and the sidebar then
never comes back when a source connects -- a worse bug than the one reported, and one
that a "was this our own write" flag does not catch either, because the toggle's signal
may fire after the write returns.

So the direction is stated instead of inferred: `show-sidebar` is an output, the button
is the only input, and `App::sidebar_wanted` holds the wish between them. The wish is
written by the toggle's handler alone (guarded while we set the button ourselves), the
no-signals case forces the panel off without touching the wish, and tick() re-asserts
both from the wish when the widget disagrees -- so a hide gesture does not stick either.

[sidebar-probe] now reads the pair (shown vs button) in both states and against a hide
that did not come from the button; shown=1 button=1 with a source, shown=0 button=0
without, and the source reconnecting brings both back is the reading this exists for.

src/ui_probe.cpp

@@ -1634,7 +1634,8 @@ gboolean boxzoom_probe(gpointer user) {
 }
 
 
-/// `UDPSCOPE_PROBE_SIDEBAR_MS`: the sidebar's width, in both of the split view's states.
+/// `UDPSCOPE_PROBE_SIDEBAR_MS`: the sidebar's width and whether the toolbar button agrees with it, in
+/// both of the split view's states and against a hide that did not come from the button.
 ///
 /// The small-window case is the one a divider position got wrong: with the sidebar's width derived from
 /// the window, a window narrower than that number gave it nothing at all. The split view takes its size
@@ -1644,24 +1645,43 @@ gboolean boxzoom_probe(gpointer user) {
 gboolean sidebar_probe(gpointer user) {
   App& a = *static_cast<App*>(user);
   if (a.side_view == nullptr) return G_SOURCE_REMOVE;
+  // `shown` is the panel's own state and `button` the toolbar's; they must agree, or the button is
+  // telling the user something the window does not do.
   const auto report = [](App& app, const char* what) {
     const int side = app.side_box != nullptr ? gtk_widget_get_width(app.side_box) : -1;
     const int win = app.window != nullptr ? gtk_widget_get_width(app.window) : -1;
-    std::fprintf(stderr, "[sidebar-probe] %-10s window=%d sidebar=%d collapsed=%d\n", what, win,
-                 side, adw_overlay_split_view_get_collapsed(ADW_OVERLAY_SPLIT_VIEW(app.side_view)) ? 1
-                                                                                                     : 0);
+    std::fprintf(stderr,
+                 "[sidebar-probe] %-10s window=%d sidebar=%d collapsed=%d shown=%d button=%d(sens=%d)\n",
+                 what, win, side,
+                 adw_overlay_split_view_get_collapsed(ADW_OVERLAY_SPLIT_VIEW(app.side_view)) ? 1 : 0,
+                 adw_overlay_split_view_get_show_sidebar(ADW_OVERLAY_SPLIT_VIEW(app.side_view)) ? 1 : 0,
+                 app.sidebar_toggle != nullptr &&
+                         gtk_toggle_button_get_active(GTK_TOGGLE_BUTTON(app.sidebar_toggle)) ? 1 : 0,
+                 app.sidebar_toggle != nullptr && gtk_widget_get_sensitive(app.sidebar_toggle) ? 1 : 0);
   };
   report(a, "beside");
   // The state a window with no room produces: the sidebar goes over the plot, and its width is still its
   // own. If this ever reads 0, the sidebar is being given a share of a width that is not there.
   adw_overlay_split_view_set_collapsed(ADW_OVERLAY_SPLIT_VIEW(a.side_view), TRUE);
+  // A hide that did not come from the button -- an overlay swipe, or the split view's own collapsing --
+  // must not win: the panel is where the button says it is. `shown` and `button` still reading 1 here is
+  // the point, and reading 0 is the regression (a button that no longer reflects the panel, or a panel
+  // that slips out from under it).
+  adw_overlay_split_view_set_show_sidebar(ADW_OVERLAY_SPLIT_VIEW(a.side_view), FALSE);
   g_timeout_add(400, +[](gpointer u) -> gboolean {
     App& app = *static_cast<App*>(u);
     const int side = app.side_box != nullptr ? gtk_widget_get_width(app.side_box) : -1;
-    std::fprintf(stderr, "[sidebar-probe] %-10s window=%d sidebar=%d collapsed=%d\n", "overlaid",
-                 app.window != nullptr ? gtk_widget_get_width(app.window) : -1, side,
-                 adw_overlay_split_view_get_collapsed(ADW_OVERLAY_SPLIT_VIEW(app.side_view)) ? 1 : 0);
+    std::fprintf(stderr,
+                 "[sidebar-probe] %-10s window=%d sidebar=%d collapsed=%d shown=%d button=%d(sens=%d)\n",
+                 "regained", app.window != nullptr ? gtk_widget_get_width(app.window) : -1, side,
+                 adw_overlay_split_view_get_collapsed(ADW_OVERLAY_SPLIT_VIEW(app.side_view)) ? 1 : 0,
+                 adw_overlay_split_view_get_show_sidebar(ADW_OVERLAY_SPLIT_VIEW(app.side_view)) ? 1 : 0,
+                 app.sidebar_toggle != nullptr &&
+                         gtk_toggle_button_get_active(GTK_TOGGLE_BUTTON(app.sidebar_toggle)) ? 1 : 0,
+                 app.sidebar_toggle != nullptr && gtk_widget_get_sensitive(app.sidebar_toggle) ? 1 : 0);
     adw_overlay_split_view_set_collapsed(ADW_OVERLAY_SPLIT_VIEW(app.side_view), FALSE);
+    // Back the way a person would: pressing the button is what records the wish, and the panel follows it.
+    gtk_toggle_button_set_active(GTK_TOGGLE_BUTTON(app.sidebar_toggle), TRUE);
     return G_SOURCE_REMOVE;
   }, &a);
   return G_SOURCE_REMOVE;