Bug 1780191. Avoid making primitives smaller than one pixel active. r=nical

Webrender doesn't seem to handle them well. This of course will affect the balance of making things active, but hopefully this is a good trade off.

Differential Revision: https://phabricator.services.mozilla.com/D165726

gfx/layers/wr/WebRenderCommandBuilder.cpp

@@ -1130,7 +1130,9 @@ static ItemActivity HasActiveChildren(
 }
 
 static ItemActivity AssessBounds(const StackingContextHelper& aSc,
-                                 const nsRect& aBounds) {
+                                 nsDisplayListBuilder* aDisplayListBuilder,
+                                 nsDisplayItem* aItem,
+                                 bool aHasActivePrecedingSibling) {
   // Arbitrary threshold up for adjustments. What we want to avoid here
   // is alternating between active and non active items and create a lot
   // of overlapping blobs, so we only make images active if they are
@@ -1139,14 +1141,28 @@ static ItemActivity AssessBounds(const StackingContextHelper& aSc,
   // concern.
   constexpr float largeish = 512;
 
-  float width = aBounds.width * aSc.GetInheritedScale().xScale;
-  float height = aBounds.height * aSc.GetInheritedScale().yScale;
+  bool snap = false;
+  nsRect bounds = aItem->GetBounds(aDisplayListBuilder, &snap);
 
-  if (width > largeish || height > largeish) {
-    return ItemActivity::Should;
+  float appUnitsPerDevPixel =
+      static_cast<float>(aItem->Frame()->PresContext()->AppUnitsPerDevPixel());
+
+  float width =
+      static_cast<float>(bounds.width) * aSc.GetInheritedScale().xScale;
+  float height =
+      static_cast<float>(bounds.height) * aSc.GetInheritedScale().yScale;
+
+  // Webrender doesn't handle primitives smaller than a pixel well, so
+  // avoid making them active.
+  if (width >= appUnitsPerDevPixel && height >= appUnitsPerDevPixel) {
+    if (aHasActivePrecedingSibling || width > largeish || height > largeish) {
+      return ItemActivity::Should;
+    }
+
+    return ItemActivity::Could;
   }
 
-  return ItemActivity::Could;
+  return ItemActivity::No;
 }
 
 // This function decides whether we want to treat this item as "active", which
@@ -1202,11 +1218,8 @@ static ItemActivity IsItemProbablyActive(
       if (StaticPrefs::gfx_webrender_svg_images() && aUniformlyScaled &&
           svgItem->ShouldBeActive(aBuilder, aResources, aSc, aManager,
                                   aDisplayListBuilder)) {
-        if (aHasActivePrecedingSibling) {
-          return ItemActivity::Should;
-        }
-        bool snap = false;
-        return AssessBounds(aSc, aItem->GetBounds(aDisplayListBuilder, &snap));
+        return AssessBounds(aSc, aDisplayListBuilder, aItem,
+                            aHasActivePrecedingSibling);
       }
 
       return ItemActivity::No;

layout/reftests/bugs/1780191-1-ref.svg

@@ -0,0 +1,36 @@
+<?xml version="1.0" standalone="no"?>
+<svg width="315px" height="445px" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+   <svg viewBox="0 0 21000 29700" id="thetarget">
+<style>
+      rect {
+            fill: rgb(187,187,187);
+      }
+</style>
+<script>
+// <![CDATA[
+function insertRect(x,y,w,h) {
+      const rect = document.createElementNS("http://www.w3.org/2000/svg", "rect");
+      rect.setAttribute("x", x.toString());
+      rect.setAttribute("y", y.toString());
+      rect.setAttribute("height", h.toString());
+      rect.setAttribute("width", w.toString());
+      document.getElementById("thetarget").appendChild(rect);      
+}
+function place() {
+      let baseleft = 2000;
+      let basetop = 2000;
+      let width = 60;
+      let height = 539;
+      let coldiff = 120;
+      let rowdiff = 600;
+      for (let row = 0; row <= 60; row++) {
+            insertRect(baseleft, basetop + rowdiff*row, 34200, height);
+      }
+}
+
+place();
+// ]]>
+</script>
+
+   </svg>
+</svg>

layout/reftests/bugs/1780191-1.svg

@@ -0,0 +1,33 @@
+<?xml version="1.0" standalone="no"?>
+<svg width="315px" height="445px" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+   <svg viewBox="0 0 21000 29700" id="thetarget">
+<script>
+// <![CDATA[
+function insertRect(x,y,w,h) {
+      const rect = document.createElementNS("http://www.w3.org/2000/svg", "rect");
+      rect.setAttribute("x", x.toString());
+      rect.setAttribute("y", y.toString());
+      rect.setAttribute("height", h.toString());
+      rect.setAttribute("width", w.toString());
+      document.getElementById("thetarget").appendChild(rect);      
+}
+function place() {
+      let baseleft = 2000;
+      let basetop = 2000;
+      let width = 18;
+      let height = 539;
+      let coldiff = 66;
+      let rowdiff = 600;
+      for (let row = 0; row <= 60; row++) {
+            for (let col = 0; col <= 360; col++) {
+                  insertRect(baseleft + col*coldiff + row, basetop + rowdiff*row, width, height);
+            }      
+      }
+}
+place();
+// ]]>
+</script>
+
+
+   </svg>
+</svg>

layout/reftests/bugs/reftest.list

@@ -2119,6 +2119,8 @@ skip-if(Android) test-pref(ui.textScaleFactor,150) test-pref(browser.display.os-
 skip-if(Android) test-pref(ui.textScaleFactor,50) test-pref(browser.display.os-zoom-behavior,1) != 1773633.html 1773633.html
 pref(widget.disable-dark-scrollbar,false) == 1777135.html 1777135-ref.html
 test-pref(widget.non-native-theme.use-theme-accent,false) == 1778834.html 1778834-ref.html
+# do not adjust the fuzz without looking, this test was written as fuzzy on purpose (because that was the only way to make a test work)
+fuzzy(16-47,26939-44425) == 1780191-1.svg 1780191-1-ref.svg
 pref(layout.css.prefers-color-scheme.content-override,0) == 1787127.html 1787127-ref.html
 pref(layout.css.prefers-color-scheme.content-override,1) == 1787127.html 1787127-ref.html
 pref(layout.css.prefers-color-scheme.content-override,2) == 1787127.html 1787127-ref.html