Here's a layer tree dump during tiled-fixed.html: BasicContainerLayer (0xd2824c00) [visible=< (x=0, y=0, w=1486, h=969); >] [opaqueContent] [metrics={ cb=(x=0.000000, y=0.000000, w=1486.000000, h=969.000000) sr=(x=0.000000, y=0.000000, w=1486.000000, h=969.000000) s=(x=0.000000, y=0.000000) dp=(x=0.000000, y=0.000000, w=0.000000, h=0.000000) cdp=(x=0.000000, y=0.000000, w=0.000000, h=0.000000) scrollId=0 z=1.000 }] BasicThebesLayer (0xd5bb4280) [visible=< (x=0, y=0, w=1486, h=969); >] [opaqueContent] [valid=< (x=0, y=0, w=1486, h=969); >] BasicContainerLayer (0xddc76400) [clip=(x=0, y=101, w=1486, h=868)] [visible=< (x=0, y=101, w=1486, h=868); >] BasicContainerLayer (0xddc7ac00) [visible=< (x=1473, y=101, w=13, h=868); >] BasicThebesLayer (0xd1dec1a0) [visible=< (x=1473, y=101, w=13, h=868); >] [opaqueContent] [valid=< (x=1473, y=101, w=13, h=868); >] BasicContainerLayer (0xddce3c00) [visible=< (x=1473, y=589, w=13, h=380); >] [vscrollbar=11] BasicThebesLayer (0xd1dec350) [transform=[ 1 0; 0 1; 1473 590; ]] [visible=< (x=0, y=-1, w=13, h=380); >] [valid=< (x=0, y=-1, w=13, h=380); >] BasicThebesLayer (0xd1deb5d0) [transform=[ 1 0; 0 1; 0 -945; ]] [visible=< (x=0, y=1046, w=1473, h=868); >] [opaqueContent] [valid=< (x=0, y=1046, w=1473, h=868); >] Obvious problem is that layer 0xddc76400 has not been marked opaque, so the chrome ThebesLayer 0xd5bb4280 covers the whole window. This probably explains some or all of the regression in bug 1042186.

Different run: ThemedBackground p=0x85aa047c f=0x914b6ea0(ScrollbarFrame(scrollbar)(-1)) bounds(88380,6060,780,52080) visible(0,0,89160,58140) componentAlpha(0,0,0,0) clip() (opaque 88380,6060,780,52080) (themed, appearance:87) layer=0x7c5c0280 OwnLayer p=0x85aa050c f=0x928eb620(ButtonBoxFrame(thumb)(0)) bounds(88380,6000,780,14400) visible(0,0,89160,58140) componentAlpha(0,0,0,0) clip() layer=0x85aa5400 ThemedBackground p=0x85aa04c4 f=0x928eb620(ButtonBoxFrame(thumb)(0)) bounds(88380,6000,780,14400) visible(0,0,89160,58140) componentAlpha(0,0,0,0) clip() (themed, appearance:89) layer=0x7c5c0430 Notice that the nsButtonBoxFrame bounds extends outside the scrollbar! That's bad.