fix: allocate space for always visible scrollbar

GTK does not allocate any additional space for scrollbars, leading to
always shown scrollbars to overlap the items in the history list.
To fix this, we manually allocate additional space when the the elements
'overflow' the screen and overlay-scrollbars are disabled.

Fixes #149

src/window.rs

@@ -331,6 +331,15 @@ impl AppWindow {
                 history_item.upcast()
             });
 
+        let gtk_settings = gtk::Settings::default().unwrap();
+        gtk_settings.connect_gtk_overlay_scrolling_notify(glib::clone!(
+            #[weak(rename_to = window)]
+            self,
+            move |settings| {
+                window.adjust_scrollbar_offset(settings);
+            }
+        ));
+
         self.history().connect_items_changed(glib::clone!(
             #[weak(rename_to = window)]
             self,
@@ -339,10 +348,30 @@ impl AppWindow {
                 let visible = items.n_items() > 1;
                 window.imp().history_list.set_visible(visible);
                 window.action_set_enabled("app.clear-history", visible);
+                window.adjust_scrollbar_offset(&gtk_settings);
             }
         ));
     }
 
+    fn adjust_scrollbar_offset(&self, settings: &gtk::Settings) {
+        // FIXME: remove this workaround once https://gitlab.gnome.org/GNOME/gtk/-/issues/6384 is fixed
+        // calculate how many history items fit into the list, before it begins scrolling
+        let screen_elements = self.imp().history_list.size(gtk::Orientation::Vertical) / 40;
+        // width has not been set yet
+        if screen_elements == 0 {
+            return;
+        }
+        let history_items = self.history().n_items();
+        let scrollbar_width =
+            if !settings.is_gtk_overlay_scrolling() && history_items >= screen_elements as u32 {
+                // size of the scrollbar
+                24
+            } else {
+                0
+            };
+        self.imp().history_list.set_margin_end(scrollbar_width);
+    }
+
     /// Save the window size when closing the window
     fn save_window_size(&self) -> Result<(), glib::BoolError> {
         let imp = self.imp();