Scrollable

[alt-tosc]

Dear treeform,

This merge request is related to #108.

I would like to have your review on this proposed implementation.
All your feedbacks are welcome: I'm not used neither to Nim, nor to contribute to public open source projects on github.

Current changes summary:

• From a user point of view: scrollable and scrollSpeed attributes added to Node, and corresponding setters.
• It supports both vertical and horizontal scrolling.
• It works with htmlbackend.
• minimal_scroll example.

Todo:

• Which kind of automated tests should I add? I think you work with automated screenshot comparison. For this feature it is a bit more complicated as there is mouse interaction and maybe one screenshot is not sufficient. Please tell me what you think :)

[treeform]

Hey, sorry I am currently working on fidget 2 - the complete rewrite. I don't know if I will be doing anything with fidget 1.

I want this to connect with scrollBars it seems like you went for a more complex system scroll system.

I think the speed of the scroll should depend OS not user setting? I don't know how to get that though.

[alt-tosc]

Hello,
I'm curious and I can't wait for your new implementation to be published :)

I have a lot to say so I'll add numbers to ease replies:

1. I want this to connect with scrollBars it seems like you went for a more complex system scroll system.

   I also tried to connect this to some scrollBars, I've considered the approach of scrollbars drawn as Fidget nodes.
   With this approach I see two advantages:

   • scrollBars are customizable
   • the rendering is uniform between desktop and browser.

2. With what I propose in this PR I have mixed feelings about scrollSpeed which is set at the level of the child node:

   • on one side it allows to easily achieve some scroll effects like parallax scroll effect, probably also 'sticky' nodes
   • on the other side it is more difficult to add scrollBars as it is more difficult to know what is inside the parent screenBox and what is outside when the scrollBar is drawn: I have the impression the scrollbar should be drawn at the end of computeLayout which also seems a bad idea.

3. I think the speed of the scroll should depend OS not user setting? I don't know how to get that though.

   I've just done a little experiment (I work on Gnome desktop): when I change scroll setting to "natural scroll", the change is reflected on the input given to the callback proc onScroll(window: staticglfw.Window, xoffset, yoffset: float64) set by window.setScrollCallback(onScroll). I don't have settings in Gnome to change scroll speed (and from a quick search it seems complicated to change on Linux desktops).

   It will probably be a good practice to not change the scrollSpeed inside Fidget and let by default scrollSpeed=1. But as mentioned earlier in point 2, scrollSpeed at the level of child node gives some interesting flexibility.

4. In some applications (like in browsers) I have the feeling the scroll speed is not linear with respect to the 'wheel displacement'. Also scrolling does not stop immediately at the end the 'wheel displacement'. But I suppose this could be added in a further iteration.

Let me know if I can help, even If I'm not the most qualified for this, I will have some free time in the next few weeks ;)