This is just an informational message to inform everyone of the design of current scrolling model. I hope this will also be useful to shine the way toward overflow:scroll. I'm also interested in hearing about potential improvements to this model, because it would be best to implement them now, before accumulating more technical debt.
Scrolling is implemented by the compositor. Compositor layers that opt in to scroll events via the wants_scroll_events flag can scroll their contents. From now on, I will refer to these as "scrolling roots." Scrolling roots serve as a viewport into their content, which is stored in descendant layers. In order for scrolling roots to be able to scroll their content, they need to be smaller than that content [1]. Since the compositor normally allows layers to exist independently of their parents, child layers can overflow or fail to intersect their parents completely. To prevent this, scrolling roots use the masks_to_bounds flag, which is a signal to the compositor that it should not paint any part of descendant layers that lies outside the boundaries of the scrolling root. Below is an (admittedly terrible) ASCII art diagram showing a scrolling root with three content layers, scrolled down a few ticks. Everything above and below the set of "====" bars would be hidden by masks_to_bounds. +-----------------------+ | | ========================= | | scrolling | <---------------+root | | | +-------+ | ========================= | | | | ++-------+ +--^----+ | || | | | || | | | content ++-------+ <----+-----------+layers | | | | | | +-----------------------+ Currently, only iframes and the main frame scroll according to this model [2]. There isn't a lot in the way to extend this model to overflow:scroll. The layout code would simply need to make a layer for the content inside the scrolling flow and a layer for the scrolling root. The two main dependencies for this would be: a. Adding a way for layers to trigger the masks_to_bounds flag. b. Adding support for building layer trees, since all layers are now added as direct children of their frame root layer. 1. If the content was larger than the scrolling root, it would not be able to move around inside the scrolling root. Imagine a browser window that is larger than the content that it contains. 2. The main frame's scroll root size is defined by the window size. --Martin _______________________________________________ dev-servo mailing list dev-servo@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-servo
