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

From: fantasai <fantasai.lists@inkedblade.net>
Date: Thu, 23 May 2013 00:53:45 +0800
Message-ID: <519CF819.9020008@inkedblade.net>
To: www-style@w3.org
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
  |                         +------+|
  +---------------------------------+

~fantasai
Received on Wednesday, 22 May 2013 16:54:18 UTC

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