bug#53839: 29.0.50; Strange bug causes tool bar to be overwritten

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

On any Motif or Lucid build, start Emacs with `-q', making sure that the
splash screen is displayed.  A single pixel at the bottom of the tool
bar will be overwritten by the window underneath it, until the frame is
resized again.

If I comment out this part of xfns.c:

      /* As long as tool_bar_resized is false, effectively try to change
	 F's native height.  */
      if (NILP (fullscreen) || EQ (fullscreen, Qfullwidth))
	adjust_frame_size (f, FRAME_TEXT_WIDTH (f), FRAME_TEXT_HEIGHT (f),
			   1, false, Qtool_bar_lines);
      else

The problem goes away, but I can't figure out why that is.

Thanks.