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Re: [css3-background] background-attachment: local

From: Simon Fraser <smfr@me.com>
Date: Wed, 29 May 2013 09:06:14 -0700
Cc: fantasai <fantasai.lists@inkedblade.net>, W3C CSS Mailing List <www-style@w3.org>
Message-id: <79C44A2A-BAFB-49A6-92E6-8266D00E8EDC@me.com>
To: Simon Sapin <simon.sapin@exyr.org>

On May 29, 2013, at 8:58 AM, Simon Sapin <simon.sapin@exyr.org> wrote:

> Le 23/05/2013 00:53, fantasai a écrit :
>> On 05/22/2013 12:27 PM, fantasai wrote:
>>> We were looking implementing 'background-attachment: local' in Gecko, and
>>> ran across the following problem in its definition:
>>> 
>>> The goal of 'background-attachment: local' is to allow authors to get
>>> the behavior of 'background-attachment: scroll' on the canvas background
>>> when setting the background of an element with 'overflow: scroll'. IMHO
>>> this should have been the behavior of 'background-attachment: scroll'
>>> in the first place, but the WG at the time decided it should behave like
>>> 'fixed' on such elements; therefore we added a new keyword, 'local', in
>>> level 3.
>>> 
>>> The spec defines this for the background's "attachment", but it doesn't
>>> for its positioning. Case to consider: a background image positioned to
>>> the bottom right corner. In the case of 'background-attachment: scroll'
>>> on the canvas, it is at the bottom of the scrollable area, not the bottom
>>> of the viewport. Similarly, 'background-attachment: local' should put
>>> such an image at the bottom of the scrollable area, not the bottom of
>>> the viewport. However, the spec doesn't specify this, so would need to
>>> be amended to fix that.
>> 
>> Here's a diagram:
>> 
>> +------------------------------------+
>> |+---------------------------------+A|
>> ||                                 | |
>> ||                                 | |
>> ||                                 | |
>> ||                                 | |
>> ||                                 | |
>> ||                                 | |
>> ||                         +------+| |
>> ||                         |+++++ || |  <-- bottom right of scrolling box
>> ||                         |+image|| |
>> ||                         +------+|V|
>> +|---------------------------------|-+
>>   |                                 |
>>   |                                 |
>>   |                                 |
>>   |                                 |
>>   |                                 |
>>   |                                 |
>>   |                                 |
>>   |                                 |
>>   |                                 |
>>   |                                 |
>>   |                         +------+|
>>   |                         |+++++ ||
>>   |                         |+image||  <-- bottom right of scrolled content
>>   |                         +------+|
>>   +---------------------------------+
> 
> Yes, I think it makes sense for the background positioning area to be in sized to the scrolled content, since this is where it is attached.

Is the size of the scrolled content area well defined?

Simon
Received on Wednesday, 29 May 2013 16:06:50 UTC

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