Backed out changeset a2078244558d (bug 1637953) as requested by :gw on slack.

gfx/wr/webrender/src/spatial_tree.rs

@@ -652,26 +652,8 @@ impl SpatialTree {
                             break;
                         }
                         ScrollFrameKind::Explicit => {
-                            // If the scroll root has no scrollable area, we don't want to
-                            // consider it. This helps pages that have a nested scroll root
-                            // within a redundant scroll root to avoid selecting the wrong
-                            // reference spatial node for a picture cache.
-                            if info.scrollable_size.width > 0.0 ||
-                               info.scrollable_size.height > 0.0 {
-                                // Since we are skipping redundant scroll roots, we may end up
-                                // selecting inner scroll roots that are very small. There is
-                                // no performance benefit to creating a slice for these roots,
-                                // as they are cheap to rasterize. The size comparison is in
-                                // local-space, but makes for a reasonable estimate. The value
-                                // is arbitrary, but is generally small enough to ignore things
-                                // like scroll roots around text input elements.
-                                if info.viewport_rect.size.width > 128.0 &&
-                                   info.viewport_rect.size.height > 128.0 {
-                                    // If we've found a root that is scrollable, and a reasonable
-                                    // size, select that as the current root for this node
-                                    scroll_root = node_index;
-                                }
-                            }
+                            // Store the explicit scroll root, keep looking up the tree
+                            scroll_root = node_index;
                         }
                     }
                 }

gfx/wr/wrench/reftests/text/text-fixed-slice.yaml

@@ -3,15 +3,15 @@
 root:
   items:
     - type: scroll-frame
-      bounds: [0, 0, 500, 200]
-      content-size: [500, 500]
+      bounds: [0, 0, 500, 100]
+      content-size: [500, 100]
       id: 2
       items:
       - type: rect
-        bounds: [0, 0, 500, 200]
+        bounds: [0, 0, 500, 100]
         color: white
     - type: rect
-      bounds: [0, 0, 500, 200]
+      bounds: [0, 0, 500, 100]
       color: white
     - text: "The sun has frightened off the night!"
       origin: 20 40

testing/web-platform/meta/css/CSS2/backgrounds/background-bg-pos-206.xht.ini

@@ -5,3 +5,5 @@
     if not webrender and (os == "win") and (version == "10.0.17134"): FAIL
     if (os == "android") and not e10s: FAIL
     if (os == "android") and e10s: PASS
+  fuzzy:
+    if webrender and (os == "win"): maxDifference=92;totalPixels=2206

testing/web-platform/meta/css/CSS2/normal-flow/min-height-106.xht.ini

@@ -1,4 +0,0 @@
-[min-height-106.xht]
-  fuzzy:
-    if os == "win" and webrender: maxDifference=92;totalPixels=5487
-

testing/web-platform/meta/css/CSS2/positioning/abspos-containing-block-001.xht.ini

@@ -2,3 +2,4 @@
   expected:
     if (os == "android") and not e10s: FAIL
     if (os == "android") and e10s: PASS
+    if webrender and (os == "win"): FAIL

testing/web-platform/meta/css/CSS2/positioning/abspos-containing-block-003.xht.ini

@@ -2,3 +2,4 @@
   expected:
     if (os == "android") and not e10s: FAIL
     if (os == "android") and e10s: PASS
+    if webrender and (os == "win"): FAIL

testing/web-platform/meta/css/filter-effects/backdrop-filter-edge-pixels.html.ini

@@ -0,0 +1,3 @@
+[backdrop-filter-edge-pixels.html]
+  fuzzy:
+    if os == "win" and webrender: maxDifference=92;totalPixels=3169