[gtk/nonoverlapping-containers: 2/2] gl: Avoid offscreens in more cases

[Matthias Clasen <matthiasc src gnome org>]

commit 73169c5048e7124a634591fb4b1a572533d0f6ab
Author: Matthias Clasen <mclasen redhat com>
Date:   Sun Apr 3 19:37:42 2022 -0400

    gl: Avoid offscreens in more cases
    
    When handling opacity, we can avoid offscreens for
    container nodes with multiple children, as long as
    they don't overlap.
    
    In particular, this avoid offscreens for ellipsized
    dim labels.

 gsk/gl/gskglrenderjob.c | 48 ++++++++++++++++++++++++++++++------------------
 1 file changed, 30 insertions(+), 18 deletions(-)
---
diff --git a/gsk/gl/gskglrenderjob.c b/gsk/gl/gskglrenderjob.c
index 30c6cd7a41..31ecb228d2 100644
--- a/gsk/gl/gskglrenderjob.c
+++ b/gsk/gl/gskglrenderjob.c
@@ -2860,8 +2860,16 @@ gsk_gl_render_job_visit_cross_fade_node (GskGLRenderJob      *job,
   gsk_gl_render_job_end_draw (job);
 }
 
+/* Figure out whether we need an offscreen to handle
+ * opacity correctly for this node. We don't if there
+ * is only one drawing node inside (could be child
+ * node, or grandchild, or...).
+ *
+ * For containers with multiple children, we can avoid
+ * the offscreen if the children are known not to overlap.
+ */
 static gboolean
-is_non_branching (const GskRenderNode *node)
+opacity_needs_offscreen (const GskRenderNode *node)
 {
   switch ((int)gsk_render_node_get_node_type (node))
     {
@@ -2877,38 +2885,46 @@ is_non_branching (const GskRenderNode *node)
     case GSK_OUTSET_SHADOW_NODE:
     case GSK_TEXT_NODE:
     case GSK_CAIRO_NODE:
-      return TRUE;
+      return FALSE;
 
     case GSK_TRANSFORM_NODE:
-      return is_non_branching (gsk_transform_node_get_child (node));
+      return opacity_needs_offscreen (gsk_transform_node_get_child (node));
 
     case GSK_OPACITY_NODE:
-      return is_non_branching (gsk_opacity_node_get_child (node));
+      return opacity_needs_offscreen (gsk_opacity_node_get_child (node));
 
     case GSK_COLOR_MATRIX_NODE:
-      return is_non_branching (gsk_color_matrix_node_get_child (node));
+      return opacity_needs_offscreen (gsk_color_matrix_node_get_child (node));
 
     case GSK_CLIP_NODE:
-      return is_non_branching (gsk_clip_node_get_child (node));
+      return opacity_needs_offscreen (gsk_clip_node_get_child (node));
 
     case GSK_ROUNDED_CLIP_NODE:
-      return is_non_branching (gsk_rounded_clip_node_get_child (node));
+      return opacity_needs_offscreen (gsk_rounded_clip_node_get_child (node));
 
     case GSK_SHADOW_NODE:
-      return is_non_branching (gsk_shadow_node_get_child (node));
+      return opacity_needs_offscreen (gsk_shadow_node_get_child (node));
 
     case GSK_BLUR_NODE:
-      return is_non_branching (gsk_shadow_node_get_child (node));
+      return opacity_needs_offscreen (gsk_shadow_node_get_child (node));
 
     case GSK_DEBUG_NODE:
-      return is_non_branching (gsk_debug_node_get_child (node));
+      return opacity_needs_offscreen (gsk_debug_node_get_child (node));
 
     case GSK_CONTAINER_NODE:
-      return gsk_container_node_get_n_children (node) == 1 &&
-             is_non_branching (gsk_container_node_get_child (node, 0));
+      if (!gsk_container_node_is_disjoint (node))
+        return TRUE;
+
+      for (int i = 0; i < gsk_container_node_get_n_children (node); i++)
+        {
+          if (opacity_needs_offscreen (gsk_container_node_get_child (node, i)))
+            return TRUE;
+        }
 
-    default:
       return FALSE;
+
+    default:
+      return TRUE;
     }
 }
 
@@ -2924,11 +2940,7 @@ gsk_gl_render_job_visit_opacity_node (GskGLRenderJob      *job,
     {
       float prev_alpha = gsk_gl_render_job_set_alpha (job, new_alpha);
 
-      /* Handle a few easy cases without offscreen. We bail out
-       * as soon as we see nodes with multiple children - in theory,
-       * we would only need offscreens for overlapping children.
-       */
-      if (is_non_branching (child))
+      if (!opacity_needs_offscreen (child))
         {
           gsk_gl_render_job_visit_node (job, child);
           gsk_gl_render_job_set_alpha (job, prev_alpha);