Non-resizable shells (e.g. DIALOG_TRIM) appear too tall when
setSize(computeSize()) is called before open() because window decoration
heights were not correctly accounted for with GTK4 CSD.

Maximized shells had their content cut off at the bottom because the
shell content was not resized to fill the full maximized area. Use the
layout signal, which fires after the the new dimensions are set, to
correctly resize the shell content to fill the maximized window.