1414103 - Enable mochitest plain [test_transformed_scrolling_repaints_2.html] on headless MacOS

Status: NEW

(Firefox :: Headless, defect, P3)

Description

[Brendan Dahl [:bdahl]]

This test currently fails with "Fully-visible scrolled element should not have been painted - got true, expected false."

I tried looking into why this fails, but haven't come up with anything conclusive yet. The call to utils.checkAndClearPaintedState goes through and looks for frames marked with NS_FRAME_PAINTED_THEBES, so I looked at why that flag is set in headless and it appears to be from an issue with the visible rect of a painted item:

Right after "t.scrollTop = 20" in FrameLayerBuilder::PaintItems() I see:

On headless:
GECKO(15745) | TILING 0x1422a8800: PaintThebes new valid region < (x=0, y=44, w=220, h=88); >
GECKO(15745) | PaintItems length=2
GECKO(15745) | PaintItems boundRect x=0 y=1199 width=12000 height=4801
GECKO(15745) | PaintItems i=0
GECKO(15745) | PaintItems GetVisibleRect x=0 y=1200 width=12000 height=1800
...PaintItem i=0 marked as NS_FRAME_PAINTED_THEBES

Normal Mode:
GECKO(15701) | TILING 0x115173000: PaintThebes new valid region < (x=0, y=44, w=220, h=88); >
GECKO(15701) | PaintItems length=2
GECKO(15701) | PaintItems boundRect x=0 y=5399 width=12000 height=601
GECKO(15701) | PaintItems i=0
GECKO(15701) | PaintItems GetVisibleRect x=0 y=0 width=0 height=0
...Here the item isn't visible so it isn't flagged