Bug 1673785 - Tweak lazy-loading root margin default to be 300px in all directions. r=hiro

This is better than 0px and such, while we wait for feedback from the
perf team.

Differential Revision: https://phabricator.services.mozilla.com/D94927

layout/base/tests/chrome/printpreview_helper.xhtml

@@ -106,8 +106,17 @@ function finish() {
   window.close();
 }
 
-function runTests()
+async function runTests()
 {
+  // This ensures we actually test the lazy-load of images in printpreview_images.
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      ["dom.image-lazy-loading.root-margin.top", 0],
+      ["dom.image-lazy-loading.root-margin.bottom", 0],
+      ["dom.image-lazy-loading.root-margin.left", 0],
+      ["dom.image-lazy-loading.root-margin.right", 0],
+    ],
+  });
   startTest1();
 }
 

modules/libpref/init/StaticPrefList.yaml

@@ -1819,7 +1819,7 @@
 # adaptative version of this.
 - name: dom.image-lazy-loading.root-margin.top
   type: float
-  value: 0
+  value: 300
   mirror: always
 
 - name: dom.image-lazy-loading.root-margin.top.percentage
@@ -1829,7 +1829,7 @@
 
 - name: dom.image-lazy-loading.root-margin.bottom
   type: float
-  value: 0
+  value: 300
   mirror: always
 
 - name: dom.image-lazy-loading.root-margin.bottom.percentage
@@ -1839,7 +1839,7 @@
 
 - name: dom.image-lazy-loading.root-margin.left
   type: float
-  value: 0
+  value: 300
   mirror: always
 
 - name: dom.image-lazy-loading.root-margin.left.percentage
@@ -1849,7 +1849,7 @@
 
 - name: dom.image-lazy-loading.root-margin.right
   type: float
-  value: 0
+  value: 300
   mirror: always
 
 - name: dom.image-lazy-loading.root-margin.right.percentage