Summary
=======

The <code>IScreenDisplay</code> interface has a <code>ScaleContents</code> property; set this to <code>true</code>

Background
==========

As [this question][1] reveals, ESRI changed the default behaviour of the map control at ArcGIS 10.

> **Less redrawing in data view**
> 
> In previous versions of ArcMap, if you changed the size of the ArcMap
> display while working in data view, either by resizing the ArcMap
> window or by docking/undocking/resizing a dockable window, *by default
> your map was completely redrawn to fit inside the available display
> area*. So the scale changed and the extent stayed the same (although
> you may have seen some extra geographic coverage based on how well the
> extent fit inside the new shape of the display area). *At version 10,
> the default drawing behavior in data view has been changed* so that
> when the display size is changed, your map is no longer completely
> redrawn to fit inside the display. Instead, the scale stays the same
> and the extent will change. If you make the display bigger, you'll see
> a larger geographic extent, and vice versa.
> 
> This has the performance advantage that the portion of the display
> unaffected by resizing doesn't need to be redrawn at all. For example,
> if you close a docked window, only the portion of the display that was
> obscured by the window needs to be redrawn. It is also easier to work
> with the display because geographic features on your map don't move
> around as you dock and undock windows. Features on your map remain in
> the same location in the display until you manually pan or zoom the
> map.

(src: http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/What_s_new_for_map_display_and_navigation/00qp0000001w000000/)


Resolution
==========

You can reverse this change by using the <code>IScreenDisplay</code> interface to set the <code>ScaleContents</code> property to <code>true</code>. This can be set once the map is loaded into the control.

For me, the line of code would read:

<code>mapControl.ActiveView.ScreenDisplay.ScaleContents = True;</code>

  [1]: http://gis.stackexchange.com/questions/146902/since-upgrading-to-arcgis-10-my-application-doesnt-rescale-the-map-when-the-wi

This restores the old map redrawing behaviour - and as a side-effect, cures the redrawing problem explained here!