Overhaul inertial scrolling +fixes

[threethan]

This is a fix to a previous pull request I made, which ended up getting mangled due to a botched rebase. The original description is as follows:

Summary

I made quite a few changes, however all are interlocked so they can't really be split to multiple branches.
I tried to be as thorough as possible with the description below, however, in short:

• Scroll behaves much nicer on mobile
• A bunch of options have been added

List of Changes

• New options have been added to project settings under a new gui>scroll section
• Completely rewrote inertial scrolling on touch. This fixes various issues, and make things far more consistent.
  • Various issues involving scroll_deadzone are now fixed. (Resolves ScrollContainer with a deadzone suddenly snaps/flickers on touch #45402)
  • Inertial scroll now decelerates quadratically instead of linear. This is more accurate to real-life friction, more consistent with other mobile applications, and subjectively feels better. It also fixes an issue where deceleration would be far too much on scaled nodes or windows.
  • The amount of inertia can be configured in project settings (gui>scroll>intertial_scroll_duration_touch)
• Added a scroll_step parameter. When scrolling with the mouse wheel, each notch will scroll by one page * scroll_step. The default is 1/8, which is the same as the previously hard-coded behavior.
  • The default value of scroll_step can be set in project settings (gui>scroll>default_scroll_step)
• When focus is within a scroll container, ui_page_up and ui_page_down input actions will now work as expected. (Previously they did nothing)
• Added a scroll_smooth parameter. This will enable Windows 10 - style smooth scrolling when scrolling using the mouse wheel, page up & down, or with follow_focus
  • The default value of scroll_smooth can be set in project settings (gui>scroll>default_scroll_smoothed)
  • The duration of the smooth scrolling animation can be set in project settings (gui>scroll>smooth_scroll_duration_button)

Changes to default behavior

I have done my best to make these changes without breaking compatibility or changing default behavior. By default only the following are changed:

• The default focus mode of a scroll_container is not CLICK instead of NONE. This is required for page up & down to work. It can manually be changed back to NONE, which revert page up & down to doing nothing, unless a child element is focused.
• Inertial scrolling will always use the new, fixed physics.
• The default_scroll_deadzone project setting is now under gui>scroll instead of gui>common
  All other new functionality requires settings to be manually changed by the user.

Context

Tested with:

• 3.2.3:
  • Windows (in editor, exported debug, exported release)
  • Android (debug, release, custom build)
• 4.0.dev:
  • Windows (in editor, debug)
  • Android (debug)

I previously submitted a similar pull request that only fixed bugs, however I closed it in favor of this one. This pull fixes all of the bugs fixed by the first one and more, by fixing the underlying issues.
This pull request does not include documentation for the new options. If/once it is merged to master, I will submit a second pull with new documentation added.
I have a 3.2 compatible version ready if desired

Bugsquad edit: This closes godotengine/godot-proposals#5386.

[threethan]

It's worth noting that, when I build from godot-latest, many options are missing from project settings. I am unsure the cause of this issue, but it has nothing to do with the changes made in this PR. It's most likely already a known issue.
This does, however, mean that the entire gui->scroll section is not visible.

[KoBeWi]

It's probably hidden under Advanced Settings.

[threethan]

It's probably hidden under Advanced Settings.

That was it, thanks! I think hiding it makes sense, so I suppose that isn't actually an issue.

[reduz]

This looks good, although I would appreciate more pictures/videos are posted so other contributors can see how it works without recompiling. Otherwise, it is difficult to tell for everyone what the differences are in look and feel by just looking at the code.

[zaksnet]

Here is a preview of this PR:

Smooth scroll OFF:

Smooth scroll ON:

And a more obvious example with smooth_scroll_animation = 1, scroll_step = 0.5:

[Calinou]

@GiantArtisan Please don't bump issues without contributing significant new information. Use the 👍 reaction button on the first post instead.

[SpyrexDE]

I've made an inofficial SmoothScroll plugin a while ago: SpyrexDE/SmoothScroll
It's still not feature-complete and looking for contributors but could be the way to go when you can't wait to get smooth scrolling into your game.
Preview of my implementation can be found on YouTube: https://youtu.be/B3GjqV2c6yQ

[allkhor]

Great job. But I see a noticeable delay with emitting "scroll_ended" signal. I'm testing this PR on the 3.x branch. The delay depends on inertial_scroll_duration_touch. With or without smooth scrolling enabled.
MRP: scroll_emmit.zip
This PR:

Old behaviour:

Can you confirm this, or maybe I was wrong when porting to the 3.x branch?

[YuriSizov]

Doesn't look like we're going to assess this for Beta 1, but we should review this and related PRs (#52128) for 4.0. This seems like an improvement many want and need.

[threethan]

Glad this is still getting looked at.
A few quick things to mention, since people seem mostly focused on the optional smoothing:

1. This still is the only fix to ScrollContainer with a deadzone suddenly snaps/flickers on touch #45402 (Well, at least that I'm aware of)
2. This makes scrolling on touch feel much closer to native (iOS/Android style) - The old/current method is very slippery, and even acts differently based on resolution
3. @allkhor found an issue where scroll_ended is delayed when using a scroll bar. Seems like dragging the scroll bar is triggering the code that handles touch screen drag scrolling. This issue isn't actually unique to this PR, theres a slight delay even in the before vid; its just longer in this PR because of scrolling physics changes. This is actually an issue with the scroll_bar widget! (Could maybe be fixed by calling _cancel_drag from the bar when it's released, but that's for another PR to deal with...)

[Maran23]

@threethan can you rebase this branch to fix the merge conflicts?

[YuriSizov]

I rebased it: #72072.
Feel free to test it and comment there.