Fixed positions

In some cases, widgets or widget content must be positioned relative to the viewport.

As in the CSS specification, these positions will be called "fixed positions". This must not be confused with "fixedly positioned elements" (see Handling Elements Out Of Flow), which are a special case of fixed positions.

Applications

As defined by CSS

• "position: fixed"; see Handling Elements Out Of Flow.
• "background-attachment: fixed"; see Images and Backgrounds in Dw.

Idea for tables

Often, tables have a header, which contains informations necessary to interpret the columns in the table body. For this, HTML defines the elements <thead> and <tbody> ^[1].

For large tables, the problem occurs that the table header gets out of the reader's view. In paged media, where a large table covers multiple pages, this is often solved by repeating the table header on each page occupied by the table. When using a viewport, a table larger than the vieport could be displayed like this:

1. If the top of the table is within the viewport, show the table header at the usual position.
2. As soon as top of the table gets above the top border of the viewport, keep the table header at the viewport top, so that it is still visible (this means, it moves down, relative to the canvas*). This way, the header is still visible, so our objective is achieved.
3. When scrolling further down, at some point the table body gets out of the viewport again, and so should the table header.

(Some images would be nice.)

These ideas should be considered when developing a design for fixed positions.

Design sketch

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^[1] ... and also <tfoot>, which is not discussed here, for reasons of simplicity. However, it is obvious that <tfoot> should be dealt with in an analogue way as <thead>.