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doc: Attempt to document the algorithm by which the Flickable interpr… (#3572)

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Co-authored-by: Olivier Goffart <olivier.goffart@slint.dev>
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docs/reference/src/language/builtins/elements.md
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@ -138,6 +138,29 @@ The maximum and preferred size of the `Flickable` are based on the viewport.
When not part of a layout, its width or height defaults to 100% of the parent
element when not specified.
### Pointer Event Interaction
If the `Flickable`'s area contains elements that use `TouchArea` to act on clicking, such as `Button`
widgets, then the following algorithm is used to distinguish between the user's intent of scrolling or
interacting with `TouchArea` elements:
1. If the `Flickable`'s `interactive` property is `false`, all events are forwarded to elements underneath.
2. Any wheel events are interpreted as attempt of the user to scroll.
3. If a press event is received where the event's coordinates interact with a `TouchArea`, the event is stored
and any subsequent move and release events are handled as follows:
1. If 100ms elapse without any events, the stored press event is delivered to the `TouchArea`.
2. If a release event is received before 100ms have elapsed, the stored press event as well as the
release event are immediately delivered to the `TouchArea` and the algorithm resets.
3. Any move events received will start a flicking operation on the `Flickable` if all of the following
conditions are met:
1. The event is received before 500ms have elapsed since receiving the press event.
2. The distance to the press event exceeds 8 logical pixels in an orientation in which we are allowed to move
If `Flickable` decides to flick, any press event sent previously to a `TouchArea`, is followed up
by an exit event. During the phase of receiving move events, the flickable follows the coordinates.
4. If the interaction of press, move, and release events begins at coordinates that do not intersect with
a `TouchArea`, then `Flickable` will flick immediately on pointer move events when the euclidean distance
to the coordinates of the press event exceeds 8 logical pixels.
### Properties
- **`interactive`** (_in_ _bool_): When true, the viewport can be scrolled by clicking on it and dragging it with the cursor. (default value: true)