Window resize annoyance

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A colleague noticed this problem today with 21.5.28 in an ESS/R context.
I've been able to reproduce it with just a normal comint shell.  To
reproduce:

    * In a fresh frame displaying one window, execute: M-x shell RET
    * Split it twice: C-x 2 C-x 2
    * Navigate to the bottom window: C-x o C-x o
    * Execute a command, e.g.: for i in 1 2 3 ; do echo $i ; done
    * Recall that command with: M-r for RET
    * Note that the window sizes changed.  Hit RET to execute the command.
    * Recall it again: M-r for RET

If your frame was tall enough the window sizes will have changed again.  It
appears to me that the middle window gets the space taken from the bottom
window.  The top window stays fixed in size.

I don't see this in 21.4.21.

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