Draw tree view rows in alternating colours ('zebra stripes')

NOTE:

Please see the README and the file `data/zebra-stripes.css` for information
on how to actually activate the zebra stripes.

6 files changed, 206 insertions, 13 deletions

diff --git a/README.md b/README.md
index 6c12d58..c5fabb4 100644
--- a/README.md
+++ b/README.md
@@ -77,6 +77,16 @@ amount of documentation about each option.
 
   **Requires disabling the GTK setting `gtk-auto-mnemonics`.**
 
+* Displays tree views with alternating row colours ('zebra stripes') if the
+  tree view contains more than one column.
+
+  <img src='screenshots/zebra-stripes.png'/>
+
+  **Requires copying the contents of the file `data/zebra-stripes.css` into
+  your `~/.config/gtk-3.0/gtk.css` file; please see the file
+  `data/zebra-stripes.css` for more information, as there is a comment at the
+  top of that file with more things you may need to know.**
+
 * Forces GTK to display normal dialog buttons at the bottom of stock dialogs
   (e.g. About dialog, color chooser, file chooser), instead of displaying the
   buttons in a headerbar at the top of the dialog -- even on Wayland.
@@ -141,11 +151,6 @@ here.
 
 * Convert menu-like popovers into traditional menus.
 
-* Restore 'zebra stripes' (alternating row colours in treeviews).
-
 ## License
 
-GNU LGPL-2.  This is the same license under which GTK 3 is released.  (This
-module used to be LGPL-3 until I was warned that GTK 3 is licensed under LGPL-2
-[*only*](https://gitlab.gnome.org/GNOME/gtk/-/blob/main/COPYING).  I didn't
-know any project other than the Linux kernel was licensed that way.)
+GNU LGPL-2.  This is the same license under which GTK 3 is released.
diff --git a/data/zebra-stripes.css b/data/zebra-stripes.css
index 5f13d79..e89c09d 100644
--- a/data/zebra-stripes.css
+++ b/data/zebra-stripes.css
@@ -1,16 +1,31 @@
-/* TODO:  This is currently unused, but when alternating treeview row colours
- * are restored, this will be the default CSS code to activate them if the
- * current theme doesn't provide its own code for this.
+/* This file enables alternating light and dark row colours ('zebra stripes')
+ * in tree view widgets.  These are general good default colours for light
+ * themes.  You can enable zebra stripes by copying this file to your
+ * ~/.config/gtk-3.0/gtk.css file, or you can implement similar code in your
+ * theme CSS.
+ *
+ * Note the extra CSS selectors which theme only tree views with more than
+ * one column; generally speaking, zebra stripes are only necessary when the
+ * list or tree contains more than one column.  If you really want zebra
+ * stripes on all tree views, regardless of column count, remove the extra
+ * selectors for ':not(.first):not(.last)', '.first:not(.last)', and
+ * ':not(.first).last'.
  */
 
 
-treeview:not(:selected).odd.sorted,
-treeview:not(:selected).even:not(.sorted)
+treeview.cell:not(.first):not(.last):not(:selected).odd.sorted,
+treeview.cell.first:not(.last):not(:selected).odd.sorted,
+treeview.cell:not(.first).last:not(:selected).odd.sorted,
+treeview.cell:not(.first):not(.last):not(:selected).even:not(.sorted),
+treeview.cell.first:not(.last):not(:selected).even:not(.sorted),
+treeview.cell:not(.first).last:not(:selected).even:not(.sorted)
 {
 	background:	alpha(black, 0.07);
 }
 
-treeview:not(:selected).even.sorted
+treeview.cell:not(.first):not(.last):not(:selected).even.sorted,
+treeview.cell.first:not(.last):not(:selected).even.sorted,
+treeview.cell:not(.first).last:not(:selected).even.sorted
 {
 	background:	alpha(black, 0.13);
 }
diff --git a/main.c b/main.c
index ddfc7ab..731b58e 100644
--- a/main.c
+++ b/main.c
@@ -14,9 +14,11 @@ void	icon_sizes_init ();
 void	no_emojis_init ();
 void	persistent_mnemonics_init ();
 void	smaller_widgets_init ();
+void	treeview_zebra_stripes_init ();
 
 static const InitFunc	init_funcs[] =
 {
+	treeview_zebra_stripes_init,
 	smaller_widgets_init,
 	/* No Emojis must be loaded before Button/Menu Icons since the former
 	 * overrides GtkMenuItem, the superclass of the GtkImageMenuItem class
diff --git a/meson.build b/meson.build
index b1417d0..4592d18 100644
--- a/meson.build
+++ b/meson.build
@@ -56,7 +56,8 @@ shared_module ('gtk3-classic-module',
 		'patches/icon-sizes.c',
 		'patches/no-emojis.c',
 		'patches/persistent-mnemonics.c',
-		'patches/smaller-widgets.c'
+		'patches/smaller-widgets.c',
+		'patches/treeview-zebra-stripes.c'
 	],
 	include_directories: [ include_dir ],
 	dependencies: [ gtk_dep, gtk_unix_print_dep ],
diff --git a/patches/treeview-zebra-stripes.c b/patches/treeview-zebra-stripes.c
new file mode 100644
index 0000000..8dafa16
--- /dev/null
+++ b/patches/treeview-zebra-stripes.c
@@ -0,0 +1,170 @@
+/* Treeview Zebra Stripes:
+ * (based on gtk3-classic patch 'treeview__alternating_row_colours.patch'):
+ *
+ * Adds appropriate style classes to each cell in a GtkTreeView, so that themes
+ * can draw the rows of a treeview in alternating colours, also known as
+ * 'zebra stripes'.
+ */
+
+
+#include <gtk/gtk.h>
+#include <gdk/gdk.h>
+#include <glib.h>
+#include <cairo/cairo.h>
+
+#include <gtk3-classic.h>
+
+
+#undef  G_LOG_DOMAIN
+#define G_LOG_DOMAIN	"Gtk3-Classic::Treeview-Zebra-Stripes"
+
+
+static gboolean	find_row_index	(GtkTreeView *	treeview,
+				 GtkTreeModel *	model,
+				 GtkTreeIter *	start_iter,
+				 GtkTreePath *	target_path,
+				 int *		index);
+
+
+INTERCEPTED_CLASS_METHOD (gtk_cell_area, render,
+			  (GtkCellArea *	area,
+			   GtkCellAreaContext *	context,
+			   GtkWidget *		widget,
+			   cairo_t *		cr,
+			   const GdkRectangle *	background_area,
+			   const GdkRectangle *	cell_area,
+			   GtkCellRendererState	flags,
+			   gboolean		paint_focus),
+			  void)
+
+
+void treeview_zebra_stripes_init ()
+{
+	GtkCellAreaClass * gtk_cell_area_class =
+	 g_type_class_ref (GTK_TYPE_CELL_AREA);
+
+	INTERCEPT_CLASS_METHOD (gtk_cell_area, GTK_CELL_AREA_CLASS,
+				render)
+
+	g_type_class_unref (gtk_cell_area_class);
+}
+
+
+static void
+new_gtk_cell_area_render
+(GtkCellArea *		area,
+ GtkCellAreaContext *	context,
+ GtkWidget *		widget,
+ cairo_t *		cr,
+ const GdkRectangle *	background_area,
+ const GdkRectangle *	cell_area,
+ GtkCellRendererState	flags,
+ gboolean		paint_focus)
+{
+	GtkTreeView *		treeview;
+	GtkTreeModel *		model;
+	GtkStyleContext *	style;
+	gint			bin_x, bin_y;
+	GtkTreePath *		path;
+	int			index;
+	GtkTreeViewColumn *	column;
+	GList *			columns;
+
+
+	if (GTK_IS_TREE_VIEW (widget))
+	{
+		treeview = GTK_TREE_VIEW (widget);
+		model = gtk_tree_view_get_model (treeview);
+		style = gtk_widget_get_style_context (widget);
+		columns = gtk_tree_view_get_columns (treeview);
+
+		bin_x = cell_area->x + cell_area->width - 1;
+		bin_y = cell_area->y + cell_area->height - 1;
+
+		gtk_tree_view_get_path_at_pos (treeview, bin_x, bin_y,
+					       &path, &column, NULL, NULL);
+		if (path != NULL)
+		{
+			index = 0;
+			find_row_index (treeview, model, NULL, path, &index);
+			gtk_tree_path_free (path);
+			gtk_style_context_add_class (style,
+			  ((index % 2) ? "even" : "odd"));
+		}
+
+		if (column == columns->data)
+			gtk_style_context_add_class (style, "first");
+		if (column == g_list_last (columns)->data)
+			gtk_style_context_add_class (style, "last");
+		if ((flags & GTK_CELL_RENDERER_SORTED))
+			gtk_style_context_add_class (style, "sorted");
+
+		gtk_render_background (style, cr,
+				       background_area->x, background_area->y,
+				       background_area->width,
+				       background_area->height);
+	}
+
+
+	CALL_ORIGINAL_CLASS_METHOD (gtk_cell_area, render,
+				    (area, context, widget, cr,
+				     background_area, cell_area,
+				     flags, paint_focus));
+}
+
+
+/* Helper functions */
+
+static gboolean
+find_row_index
+(GtkTreeView *	treeview,
+ GtkTreeModel *	model,
+ GtkTreeIter *	start_iter,
+ GtkTreePath *	target_path,
+ int *		index)
+{
+	GtkTreeIter	iter;
+	GtkTreeIter	child_iter;
+	GtkTreePath *	path;
+
+
+	if (start_iter == NULL)
+	{
+		if (! gtk_tree_model_get_iter_first (model, &iter))
+			return TRUE;
+	}
+	else
+		iter = *start_iter;
+
+	while (1)
+	{
+		path = gtk_tree_model_get_path (model, &iter);
+
+		if (gtk_tree_path_compare (path, target_path) == 0)
+		{
+			gtk_tree_path_free (path);
+			return TRUE;
+		}
+
+		if (gtk_tree_view_row_expanded (treeview, path))
+		{
+			(*index)++;
+			gtk_tree_model_iter_children (model, &child_iter, &iter);
+			if (find_row_index (treeview, model, &child_iter,
+					    target_path, index))
+			{
+				gtk_tree_path_free (path);
+				return TRUE;
+			}
+		}
+
+		gtk_tree_path_free (path);
+		if (! gtk_tree_model_iter_next (model, &iter))
+			break;
+		else
+			(*index)++;
+	}
+
+
+	return FALSE;
+}
diff --git a/screenshots/zebra-stripes.png b/screenshots/zebra-stripes.png
new file mode 100644
index 0000000..c6a9afb
--- /dev/null
+++ b/screenshots/zebra-stripes.png