theme settings: Don't use an additional stack for custom mode.

- Make 'simplified' page a sibling to the advanced pages, but leave it
  untitled, which keeps it invisible to the stack switcher.
- Hide the switcher when the simplified page is visible.
- Keep using crossfade to switch between simple and advanced views, keeping
  the left/right fade between advanced mode pages.

Fixes linuxmint/mint21.2-beta#20

files/usr/share/cinnamon/cinnamon-settings/modules/cs_themes.py

@@ -186,7 +186,7 @@ def on_module_selected(self):
             builder = Gtk.Builder()
             builder.set_translation_domain('cinnamon')
             builder.add_from_file(gladefile)
-            page = builder.get_object("page_themes")
+            page = builder.get_object("page_simplified")
             page.show()
 
             self.style_combo = builder.get_object("style_combo")
@@ -197,8 +197,6 @@ def on_module_selected(self):
             self.customize_button = builder.get_object("customize_button")
             self.preset_button = builder.get_object("preset_button")
             self.color_label = builder.get_object("color_label")
-            self.main_stack = builder.get_object("main_stack")
-            self.custom_stack = builder.get_object("custom_stack")
             self.active_style = None
             self.active_mode_name = None
             self.active_variant = None
@@ -216,19 +214,16 @@ def on_module_selected(self):
 
             self.reset_look_ui()
 
-            if self.active_variant is None:
-                self.main_stack.set_visible_child_name("custom_page")
-
             self.mixed_button.connect("clicked", self.on_mode_button_clicked, "mixed")
             self.dark_button.connect("clicked", self.on_mode_button_clicked, "dark")
             self.light_button.connect("clicked", self.on_mode_button_clicked, "light")
             self.customize_button.connect("clicked", self.on_customize_button_clicked)
             self.style_combo.connect("changed", self.on_style_combo_changed)
 
-            self.sidePage.stack.add_named(page, "themes")
+            self.sidePage.stack.add_named(page, "simplified")
 
             page = SettingsPage()
-            self.custom_stack.add_titled(page, "themes", _("Themes"))
+            self.sidePage.stack.add_titled(page, "themes", _("Themes"))
 
             settings = page.add_section()
 
@@ -247,14 +242,14 @@ def on_module_selected(self):
             button = Gtk.Button()
             button.set_label(_("Simplified settings..."))
             button.set_halign(Gtk.Align.END)
-            button.connect("clicked", self.on_preset_button_clicked)
+            button.connect("clicked", self.on_simplified_button_clicked)
             page.add(button)
 
             page = DownloadSpicesPage(self, 'theme', self.spices, self.window)
-            self.custom_stack.add_titled(page, 'download', _("Add/Remove"))
+            self.sidePage.stack.add_titled(page, 'download', _("Add/Remove"))
 
             page = SettingsPage()
-            self.custom_stack.add_titled(page, "options", _("Settings"))
+            self.sidePage.stack.add_titled(page, "options", _("Settings"))
 
             settings = page.add_section(_("Miscellaneous options"))
 
@@ -340,6 +335,8 @@ def on_module_selected(self):
 
             self.refresh_choosers()
 
+            GLib.idle_add(self.set_simplified_mode, self.active_variant is not None, True)
+
     def is_variant_active(self, variant):
         # returns whether or not the given variant corresponds to the currently selected themes
         if variant.gtk_theme != self.settings.get_string("gtk-theme"):
@@ -519,11 +516,26 @@ def on_customize_button_clicked(self, button):
         self.set_button_chooser(self.cursor_chooser, self.settings.get_string("cursor-theme"), 'icons', 'cursors', 32)
         self.set_button_chooser(self.theme_chooser, self.settings.get_string("gtk-theme"), 'themes', 'gtk-3.0', 35)
         self.set_button_chooser(self.cinnamon_chooser, self.cinnamon_settings.get_string("name"), 'themes', 'cinnamon', 60)
-        self.main_stack.set_visible_child_name("custom_page")
+        self.set_simplified_mode(False)
 
-    def on_preset_button_clicked(self, button):
+    def on_simplified_button_clicked(self, button):
         self.reset_look_ui()
-        self.main_stack.set_visible_child_name("preset_page")
+        self.set_simplified_mode(True)
+
+    def set_simplified_mode(self, simplified, startup=False):
+        # When picking a start page at startup, no transition, or else you'll see the tail end of it happening
+        # as the page is loading. Otherwise, crossfade when switching between simple/custom. The left/right
+        # transition is kept as the default for shifting between the 3 custom pages (themes, downloads, settings).
+        if simplified:
+            self.sidePage.stack.set_visible_child_full("simplified", Gtk.StackTransitionType.CROSSFADE)
+            Gio.Application.get_default().stack_switcher.set_opacity(0.0)
+        else:
+            if startup:
+                self.sidePage.stack.set_visible_child_full("themes", Gtk.StackTransitionType.NONE)
+            else:
+                self.sidePage.stack.set_visible_child_full("themes", Gtk.StackTransitionType.CROSSFADE)
+
+            Gio.Application.get_default().stack_switcher.set_opacity(1.0)
 
     def on_color_button_clicked(self, button, variant):
         print("Color button clicked")