PageScrollSize and ScrollSize are hard coded values

[tomlm]

AvaloniaEdit uses hard coded SmallChange and LargeChange values for TextArea.

TextView.cs line 2095

        Size ILogicalScrollable.ScrollSize => new Size(10, 50);

        Size ILogicalScrollable.PageScrollSize => new Size(10, 100);

2 issues:

• When using AvaloniaEdit on Consolonia these are huge values, as 1 pixel = 1 character, so small scroll is scrolling 50 lines and large is 100 lines!
• It's not changeable or configurable in any way I can figure out.

Expected:
That scrolling size is a settable property, either in options or on the textview itself.

[tomlm]

BTW: I was able to come up with a hack workaround.

I defined my own style for Scrollbars:

 <Style Selector="^:vertical">
     <Setter Property="SmallChange"
             Value="1" />
     <Setter Property="LargeChange"
             Value="{Binding Viewport, RelativeSource={RelativeSource AncestorType=ScrollViewer},Converter={StaticResource SizeToHeightConverter}}"/>
 </Style>
 <Style Selector="^:horizontal">
     <Setter Property="SmallChange"
             Value="1" />
     <Setter Property="LargeChange"
             Value="{Binding Viewport, RelativeSource={RelativeSource AncestorType=ScrollViewer}, Converter={StaticResource SizeToWidthConverter}}"/>
 </Style>

With a Size converters to get appropriate values. This seems to work, but I worry about perf and whether I'm messing up ILogicalScrollable behavior.

[tomlm]

Related to this, ScrollContentPresenter OnPointerWheelChanged has hard coded values of 50 if it's not a ILogicalScrollable
ScrollContentPresenter line 650

 
                if (Extent.Height > Viewport.Height)
                {
                    double height = isLogical ? scrollable!.ScrollSize.Height : 50;
...
                }

                if (Extent.Width > Viewport.Width)
                {
                    double width = isLogical ? scrollable!.ScrollSize.Width : 50;
..
                }

This seems like it should be relative to default font metrics, for example something like
textMetrics.LineHeight * 4;
or something

[danipen]

I think it's worth to take a look at the upstream AvalonEdit implementation and see what they are doing and maybe try to mimic the behaviour there.

[tomlm]

It looks like AvalonEdit/WPF uses IScrollInfo which defers to the content for all of these calculations:

method	description
LineDown()	Scrolls down within content by one logical unit.
LineLeft()	Scrolls left within content by one logical unit.
LineRight()	Scrolls right within content by one logical unit.
LineUp()	Scrolls up within content by one logical unit.
MakeVisible(Visual, Rect)	Forces content to scroll until the coordinate space of a Visual object is visible.
MouseWheelDown()	Scrolls down within content after a user clicks the wheel button on a mouse.
MouseWheelLeft()	Scrolls left within content after a user clicks the wheel button on a mouse.
MouseWheelRight()	Scrolls right within content after a user clicks the wheel button on a mouse.
MouseWheelUp()	Scrolls up within content after a user clicks the wheel button on a mouse.
PageDown()	Scrolls down within content by one page.
PageLeft()	Scrolls left within content by one page.
PageRight()	Scrolls right within content by one page.
PageUp()	Scrolls up within content by one page.
SetHorizontalOffset(Double)	Sets the amount of horizontal offset.
SetVerticalOffset(Double)	Sets the amount of vertical offset.

[tomlm]

Looks like they do what I suggest:

		void IScrollInfo.MouseWheelDown()
		{
			((IScrollInfo)this).SetVerticalOffset(
				scrollOffset.Y + (SystemParameters.WheelScrollLines * DefaultLineHeight));
			OnScrollChange();
		}

		void IScrollInfo.MouseWheelLeft()
		{
			((IScrollInfo)this).SetHorizontalOffset(
				scrollOffset.X - (SystemParameters.WheelScrollLines * WideSpaceWidth));
			OnScrollChange();
		}

They map WheelScroll delta to DefaultLineHeight or for horizontal to CharWidth.

(That's avalonEdit TextView.cs code)

https://github.com/icsharpcode/AvalonEdit/blob/862415d51eddc9eac93f462dbc522ffbf929cd52/ICSharpCode.AvalonEdit/Rendering/TextView.cs#L4