Bug 1650438 - Don't expand the dirty rect for fixed items during a pa…

…rtial update. r=mstange,botond

When doing a display list build, there's some code that expands the dirty
and visible rects for out-of-flow items to include the entire visual viewport
or displayport, because out-of-flow items are specially positioned in the
compositor and may become visible without the main thread knowing about it.

However, this happens even during partial updates using retained display lists,
which I believe is undesirable because the preceding non-partial update should
already have painted all the right things. The partial update should be
restricted to the part of the OOF item that actually need repainting, and the
merging of the partial update into the full display list should produce a final
display list that covers everything.

Differential Revision: https://phabricator.services.mozilla.com/D82216

gfx/layers/apz/test/mochitest/test_group_zoom.html

@@ -44,6 +44,14 @@
   ["apz.max_tap_time", 10000],
 ];
 
+// For helper_fixed_pos_displayport the mechanism we use to record the
+// fixed-pos displayport only takes effect when not in a partial display list
+// update. So for this test we just disable retained display lists entirely.
+var no_rdl_prefs = [
+  ...prefs,
+  ["layout.display-list.retain", false],
+];
+
 var subtests = [
   {"file": "helper_basic_zoom.html", "prefs": prefs},
   {"file": "helper_basic_onetouchpinch.html", "prefs": onetouchpinch_prefs},
@@ -54,7 +62,7 @@
   {"file": "helper_visual_smooth_scroll.html", "prefs": prefs},
   {"file": "helper_scroll_into_view_bug1516056.html", "prefs": prefs},
   {"file": "helper_scroll_into_view_bug1562757.html", "prefs": prefs},
-  {"file": "helper_fixed_pos_displayport.html", "prefs": prefs},
+  {"file": "helper_fixed_pos_displayport.html", "prefs": no_rdl_prefs},
   // If you're adding more tests, add them to test_group_zoom-2.html
 ];
 

layout/painting/nsDisplayList.cpp

@@ -440,9 +440,11 @@ nsRect nsDisplayListBuilder::OutOfFlowDisplayData::ComputeVisibleRectForFrame(
   nsRect visible = aVisibleRect;
   nsRect dirtyRectRelativeToDirtyFrame = aDirtyRect;
 
+  bool inPartialUpdate =
+      aBuilder->IsRetainingDisplayList() && aBuilder->IsPartialUpdate();
   if (StaticPrefs::apz_allow_zooming() &&
       nsLayoutUtils::IsFixedPosFrameInDisplayPort(aFrame) &&
-      aBuilder->IsPaintingToWindow()) {
+      aBuilder->IsPaintingToWindow() && !inPartialUpdate) {
     dirtyRectRelativeToDirtyFrame =
         nsRect(nsPoint(0, 0), aFrame->GetParent()->GetSize());