62543 - overflow:hidden not working on Linux with transparent GIFs

Status: RESOLVED DUPLICATE of bug 78497

(Core :: Web Painting, defect, P3)

Description

[Simon Lodal]

BuildID: 2000120808
UserAgent: Mozilla/5.0 (X11; U; Linux 2.2.17 i686; en-US; m18) Gecko/20001208


DESCRIPTION
An absolutely positioned div has an image in it. The div has "overflow:hidden".
Too, it either has width and height set, or it has clip:rect(...) set.


EXPECTED BEHAVIOUR
The div should be clipped, and the contained image should also be clipped so it
always fits inside the visible part of the div.


ACTUAL BEHAVIOUR
Under Linux the image is not clipped at all. The div's background color is
clipped. Thus overflow:hidden seems to affect the div itself, but not it's
children. Things work as expected on Win32. Not tested on Mac.


DETAILS
The bug only happens when the image is a GIF with transparency. Non-transparent
GIF's and JPEG's seem to not trigger the bug. Not tested with other image formats.

There are two ways to clip the div, when "overflow:hidden" is set. Both of them
work under Win32. The following is what happens under Linux:
1) Setting "clip:rect(0px, 18px, 18px, 0px)" (top,right,bottom,left order). This
simply does not work at all.
2) Setting "width:18px; height:18px". This triggers strange behaviour: When
loading the page the image is initially clipped, though not entirely correct, it
seems to be clipped on 19 pixels height instead of 18. Only after refreshing the
browser window (shade and unshade is enough; no need for resize) the entire
image becomes visible. I have seen other display artifacts than the 19'th
scanline, those however might be another bug, since they only occur with lots of
nested divs and tables, and not in this test case (investigating this further).


TEST CASES
As simple as I could make them:
http://www.mirrormind.com/tests/mozilla/clip.html
http://www.mirrormind.com/tests/mozilla/width_height.html


NOTES
Probably this bug belongs to other (probably more) components; as a programmer I
feel indications of something more than just a clipping issue:
- How can it make a difference if the image is transparent or not? Is this
something with image handling in general?
- Why does the width/height case only happen after refreshing the display? Could
it be that clipping is correct, but display refresh trashes clipping?
- Finally, what does clipping have to do with Linux and Win32, clipping ought to
be completely platform independent?

However I don't know which components to assign this to otherwise, and I've
already spent way too much time trying to figure out this bugzilla stuff.

Comment 1

[Hixie (not reading bugmail)]

Isn't |rect(0px, 18px, 18px, 0px)| an invalid rect? I'm not sure, I can never
remember these things.

Sending to views for confirmation and triage.

Comment 2

[Keyser Sose]

I am seeing what is described in #2 under Details
Platform: PC
OS: Linux 2.2.16
Mozilla Build: 2000121908 M18 Trunk Build

Very good bug report BTW. 

Marking as NEW.

Comment 3

[Kevin McCluskey (gone)]

From CSS2 Errata:

While CSS2 specifies that values of "rect()" specify offsets from the respective
sides of the box, current implementations interpret values with respect to the
top and left edges for all four values (top, right, bottom, and left). The
Working Group proposes to revise CSS2 to conform to current practice. 

Comment 4

[Kevin McCluskey (gone)]

Setting milestone to Mozilla1.0.1

Comment 5

[Kevin McCluskey (gone)]

Clearing milestone.

Comment 6

[Kevin McCluskey (gone)]

Don, At some point we need to handle clipping of transparent images.

Comment 7

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

*** This bug has been marked as a duplicate of 78497 ***