786100 - componentAlpha unnecessarily used when scrolling text over constant background color

Status: RESOLVED FIXED

(Core :: Layout, defect)

Description

[Karl Tomlinson (:karlt)]

Attachment: testcase

STR:
1) Run debug browser with NSPR_LOG_MODULES=Layers:5 in environment
2) load testcase
3) click in content area
4) use up/down keys to scroll

Actual results: Output includes "[componentAlpha]"

Expected: no "[componentAlpha]"

Comment 1

[Karl Tomlinson (:karlt)]

Need to debug from FindOpaqueBackgroundColorFor().

Comment 2

[Karl Tomlinson (:karlt)]

Need layers.acceleration.force-enabled to demonstrate this way, probably since http://hg.mozilla.org/integration/mozilla-inbound/rev/7a98f847bb8e

Comment 3

[Karl Tomlinson (:karlt)]

It seems the uniform color area is 2/15 pixel short of the top of the scrolled region (and extends 2/15 pixel below).

I wonder whether the the display item hasn't been snapped to pixels appropriately.  It looks like it is drawn snapped to pixels.

In FindOpaqueColorCovering, at http://hg.mozilla.org/mozilla-central/file/1489b6c2d1d2/layout/base/FrameLayerBuilder.cpp#l2559

(gdb) p aRect
$33 = (
    const nsRect &) @0x7fff100bc6a0: {<mozilla::gfx::BaseRect<int, nsRect, nsPoint, nsSize, nsMargin>> = {x = 480, y = 5700, width = 16800, 
    height = 12000}, <No data fields>}

(gdb) p visible
$45 = (
    const nsRect &) @0x1ba6678: {<mozilla::gfx::BaseRect<int, nsRect, nsPoint, nsSize, nsMargin>> = {x = 480, y = 5708, width = 17100, 
    height = 12000}, <No data fields>}
(gdb) p item
$46 = (nsDisplayBackgroundColor *) 0x1ba6650

(gdb) p i
$48 = 1

(gdb) p /x item->mColor
$50 = 0xff808080

Comment 4

[Karl Tomlinson (:karlt)]

Even the nsDisplayCanvasBackground is 2/15 pixel out of alignment.

Comment 5

[Karl Tomlinson (:karlt)]

The off-pixel alignment comes from the TextBox for the menubar being 15 + 2/15 pixels high.

MenuBar(menubar)(0)@0x248c6f8 {0,0,20880,1268} [state=0000160080c40000] [content=0x1ab6b60] [sc=0x26022b0]<
  Menu(menu)(0)@0x24ae4b8 next=0x24aec20 {0,0,2040,1268} [state=0000164090540010] [content=0x1ab72b0] [sc=0x25c48a8]<
    TextBox(label)(-1)[value=File]@0x24aea68 {420,180,1200,908} [state=0000064000000010] [content=0x248f940] [sc=0x2602028]
  >
  PopupList<
[...]

That's perhaps not ideal, but not the real bug here.

Comment 6

[Robert O'Callahan (:roc) (email my personal email if necessary)]

(In reply to Karl Tomlinson (:karlt) from comment #3)
> It seems the uniform color area is 2/15 pixel short of the top of the
> scrolled region (and extends 2/15 pixel below).
> 
> I wonder whether the the display item hasn't been snapped to pixels
> appropriately.  It looks like it is drawn snapped to pixels.

Yes, it sounds like something needs to be snapped. The display item's GetBounds should return true for aSnap in GetBounds, and then the FrameLayerBuilder should be using that snapped rect after IsUniform returns true.

Comment 7

[Karl Tomlinson (:karlt)]

Attachment: consider display item snap

Comment 8

[Karl Tomlinson (:karlt)]

https://tbpl.mozilla.org/?tree=Try&rev=7a5ff1fd58f1

Comment 9

[Karl Tomlinson (:karlt)]

https://hg.mozilla.org/integration/mozilla-inbound/rev/3399897304da

Comment 10

[Ed Morley [:emorley]]

https://hg.mozilla.org/mozilla-central/rev/3399897304da