The EWMH specification doesn't say anything about sending client messages. Here is the relevant portion:
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_NET_WORKAREA
_NET_WORKAREA, x, y, width, height CARDINAL[][4]/32
This property MUST be set by the Window Manager upon calculating the work area for each desktop. Contains a geometry for each desktop. These geometries are specified relative to the viewport on each desktop and specify an area that is completely contained within the viewport. Work area SHOULD be used by desktop applications to place desktop icons appropriately.
The Window Manager SHOULD calculate this space by taking the current page minus space occupied by dock and panel windows, as indicated by the _NET_WM_STRUT or _NET_WM_STRUT_PARTIAL properties set on client windows.
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However, he's right that we're not updating it when the desktop size changes. I'll fix that right away.