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horizontal-scrolling-and-virtualization.md

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Horizontal scrolling and virtualization

Scope

Minor

Version Introduced

4.6.2

Source Analyzer Status

Planned

Change Description

This change applies to an xref:System.Windows.Controls.ItemsControl?displayProperty=name that does its own virtualization in the direction orthogonal to the main scrolling direction (the chief example is xref:System.Windows.Controls.DataGrid?displayProperty=name with EnableColumnVirtualization="True"). The outcome of certain horizontal scrolling operations has been changed to produce results that are more intuitive and more analogous to the results of comparable vertical operations.

The operations include "Scroll Here" and "Right Edge", to use the names from the menu obtained by right-clicking a horizontal scrollbar. Both of these compute a candidate offset and call xref:System.Windows.Controls.Primitives.IScrollInfo.SetHorizontalOffset(System.Double).

After scrolling to the new offset, the notion of "here" or "right edge" may have changed because newly de-virtualized content has changed the value of xref:System.Windows.Controls.Primitives.IScrollInfo.ExtentWidth?displayProperty=name.

Prior to .NET Framework 4.6.2, the scroll operation simply uses the candidate offset, even though it may not be "here" or at the "right edge" any more. This results in effects like "bouncing" the scroll thumb, best illustrated by example. Suppose a xref:System.Windows.Controls.DataGrid?displayProperty=name has ExtentWidth=1000 and Width=200. A scroll to "Right Edge" uses candidate offset 1000 - 200 = 800. While scrolling to that offset, new columns are de- virtualized; let's suppose they are very wide, so that the xref:System.Windows.Controls.Primitives.IScrollInfo.ExtentWidth?displayProperty=name changes to 2000. The scroll ends with HorizontalOffset=800, and the thumb "bounces" back to near the middle of the scrollbar - precisely at 800/2000 = 40%.

The change is to recompute a new candidate offset when this situation occurs, and try again. (This is how vertical scrolling works already.)

The change produces a more predictable and intuitive experience for the end user, but it could also affect any app that depends on the exact value of xref:System.Windows.Controls.Primitives.IScrollInfo.HorizontalOffset?displayProperty=name after a horizontal scroll, whether invoked by the end user or by an explicit call to xref:System.Windows.Controls.Primitives.IScrollInfo.SetHorizontalOffset(System.Double).

  • Quirked
  • Build-time break

Recommended Action

An app that uses a predicted value for xref:System.Windows.Controls.Primitives.IScrollInfo.HorizontalOffset?displayProperty=name should be changed to fetch the actual value (and the value of xref:System.Windows.Controls.Primitives.IScrollInfo.ExtentWidth?displayProperty=name) after any horizontal scroll that could change xref:System.Windows.Controls.Primitives.IScrollInfo.ExtentWidth?displayProperty=name due to de-virtualization.

Affected APIs

  • T:System.Windows.Controls.Primitives.IScrollInfo

Category

Windows Presentation Foundation (WPF)