[verified: EXPLORER] does not perform reactivate title bar on right-button-up on an already activated pane in taskswnd

ros r74969
attention: our explorer behaves different here on ros and Windows and does it wrong differently on both systems. For now let's just try to fix the following when ros explorer is run without rshell on Windows XP.

Repro:
-Open one Window (let's take cmd again)
-window title bar and taskwnd pane should be activated
-slowly press the right mouse button on the already activated pane (and do not release it yet!)
Expected behavior:
-Title bar gets inactive (all windows are inactive), pane stays in pressed state

-release the button again:
Expected behavior:
-Title bar gets reactivated, context menu opens, pane stays in pressed state
Observed behavior (our explorer without rshell on Win):
-Title stays inactive, context menu opens, pane stays in pressed state

The title-bar staying inactive is all that needs to be fixed to close this ticket.
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The rest below is just for better understanding (where we might have additional hacks currently in ros):

When we perform this sequence with our explorer on ros, it behaves completely different:

-on r button down Title bar stays active erroneously, pane stays in pressed state
-on r button up it 'seems' ok again: Title bar is still active afterwards, context menu opens, pane stays in pressed state.

I conclude:
-explorer implementation misses to trigger title-bar reactivation on RbuttonUp
-some other part of ros misses the title-bar deactivation on RbuttonDown

Fix first one is enough to fix this ticket for now.