[gnome-shell] st/scroll-view-fade: Add :extend-fade-area property

[Marge Bot <marge-bot src gnome org>]

commit 3eda6729296591c46cb87ac86414f846c835ec2b
Author: Florian Müllner <fmuellner gnome org>
Date:   Mon May 3 19:39:06 2021 +0200

    st/scroll-view-fade: Add :extend-fade-area property
    
    Since commit f60a469a34e, everything outside the fade area is painted
    fully transparent. That is required for the fade effect during app
    grid navigation, to prevent unfaded parts of surrounding pages
    becoming visible on wide-screen displays.
    
    However in most other cases, that behavior is the exact opposite
    of what we want: Elements outside the fade area (like scroll bars)
    should never fade.
    
    In order to fix the regular case, hide the new behavior behind a
    property.
    
    https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4234
    
    Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1831>

 src/st/st-scroll-view-fade.c    | 36 ++++++++++++++++++++++++++++++++++++
 src/st/st-scroll-view-fade.glsl | 16 +++++++++++-----
 2 files changed, 47 insertions(+), 5 deletions(-)
---
diff --git a/src/st/st-scroll-view-fade.c b/src/st/st-scroll-view-fade.c
index b387e384a5..6c0176f5d9 100644
--- a/src/st/st-scroll-view-fade.c
+++ b/src/st/st-scroll-view-fade.c
@@ -45,6 +45,7 @@ struct _StScrollViewFade
   StAdjustment *hadjustment;
 
   guint fade_edges : 1;
+  guint extend_fade_area: 1;
 
   ClutterMargin fade_margins;
 };
@@ -58,6 +59,7 @@ enum {
 
   PROP_FADE_MARGINS,
   PROP_FADE_EDGES,
+  PROP_EXTEND_FADE_AREA,
 
   N_PROPS
 };
@@ -168,6 +170,7 @@ st_scroll_view_fade_paint_target (ClutterOffscreenEffect *effect,
                                      value <= 1.0 :
                                      (rtl ? value > 0.0 : value < 1.0));
 
+  clutter_shader_effect_set_uniform (shader, "extend_fade_area", G_TYPE_INT, 1, self->extend_fade_area);
   clutter_shader_effect_set_uniform (shader, "fade_offset_top", G_TYPE_FLOAT, 1, ABS 
(self->fade_margins.top));
   clutter_shader_effect_set_uniform (shader, "fade_offset_bottom", G_TYPE_FLOAT, 1, ABS 
(self->fade_margins.bottom));
   clutter_shader_effect_set_uniform (shader, "fade_offset_left", G_TYPE_FLOAT, 1, ABS 
(self->fade_margins.left));
@@ -325,6 +328,21 @@ st_scroll_view_fade_set_fade_edges (StScrollViewFade *self,
   g_object_thaw_notify (G_OBJECT (self));
 }
 
+static void
+st_scroll_view_fade_set_extend_fade_area (StScrollViewFade *self,
+                                          gboolean          extend_fade_area)
+{
+  if (self->extend_fade_area == extend_fade_area)
+    return;
+
+  self->extend_fade_area = extend_fade_area;
+
+  if (self->actor != NULL)
+    clutter_actor_queue_redraw (self->actor);
+
+  g_object_notify_by_pspec (G_OBJECT (self), props[PROP_EXTEND_FADE_AREA]);
+}
+
 static void
 st_scroll_view_fade_set_property (GObject *object,
                                   guint prop_id,
@@ -341,6 +359,9 @@ st_scroll_view_fade_set_property (GObject *object,
     case PROP_FADE_EDGES:
       st_scroll_view_fade_set_fade_edges (self, g_value_get_boolean (value));
       break;
+    case PROP_EXTEND_FADE_AREA:
+      st_scroll_view_fade_set_extend_fade_area (self, g_value_get_boolean (value));
+      break;
     default:
       G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
       break;
@@ -363,6 +384,9 @@ st_scroll_view_fade_get_property (GObject *object,
     case PROP_FADE_EDGES:
       g_value_set_boolean (value, self->fade_edges);
       break;
+    case PROP_EXTEND_FADE_AREA:
+      g_value_set_boolean (value, self->extend_fade_area);
+      break;
     default:
       G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
       break;
@@ -414,6 +438,18 @@ st_scroll_view_fade_class_init (StScrollViewFadeClass *klass)
                           FALSE,
                           ST_PARAM_READWRITE);
 
+  /**
+   * StScrollViewFade:extend-fade-area:
+   *
+   * Whether faded edges should extend beyond the faded area of the #StScrollViewFade.
+   */
+  props[PROP_EXTEND_FADE_AREA] =
+    g_param_spec_boolean ("extend-fade-area",
+                          "Extend Fade Area",
+                          "Whether faded edges should extend beyond the faded area",
+                          FALSE,
+                          ST_PARAM_READWRITE);
+
   g_object_class_install_properties (gobject_class, N_PROPS, props);
 }
 
diff --git a/src/st/st-scroll-view-fade.glsl b/src/st/st-scroll-view-fade.glsl
index 0d8812b078..39ed6a3450 100644
--- a/src/st/st-scroll-view-fade.glsl
+++ b/src/st/st-scroll-view-fade.glsl
@@ -28,6 +28,7 @@ uniform bool  fade_edges_top;
 uniform bool  fade_edges_right;
 uniform bool  fade_edges_bottom;
 uniform bool  fade_edges_left;
+uniform bool  extend_fade_area;
 
 uniform vec2 fade_area_topleft;
 uniform vec2 fade_area_bottomright;
@@ -38,11 +39,11 @@ void main ()
 
     float y = height * cogl_tex_coord_in[0].y;
     float x = width * cogl_tex_coord_in[0].x;
+    float ratio = 1.0;
 
     if (x > fade_area_topleft[0] && x < fade_area_bottomright[0] &&
         y > fade_area_topleft[1] && y < fade_area_bottomright[1])
     {
-        float ratio = 1.0;
         float fade_top_start = fade_area_topleft[1] + fade_offset_top;
         float fade_left_start = fade_area_topleft[0] + fade_offset_left;
         float fade_bottom_start = fade_area_bottomright[1] - fade_offset_bottom;
@@ -67,9 +68,14 @@ void main ()
         if (fade_right) {
             ratio *= (fade_area_bottomright[0] - x) / (fade_area_bottomright[0] - fade_right_start);
         }
-
-        cogl_color_out *= ratio;
-    } else {
-        cogl_color_out *= 0.0;
+    } else if (extend_fade_area) {
+        if (x <= fade_area_topleft[0] && fade_edges_left ||
+            x >= fade_area_bottomright[0] && fade_edges_right ||
+            y <= fade_area_topleft[1] && fade_edges_top ||
+            y >= fade_area_bottomright[1] && fade_edges_bottom) {
+            ratio = 0.0;
+        }
     }
+
+    cogl_color_out *= ratio;
 }