I took a look at the somewhat messy popup placement in IDLE.  The query.Query subclasses are centered over their parent.  Some others are over the window but up and to the left.  Maybe they have a custom geometry setting that I should delete.  Search and replace dialogs are at the upper left of the screen.  Bad if window is at the right edge.  Editor SyntaxError boxes, with no placement specified, are centered on the screen.  Centered on the text frame would be much better (as long as it did not cover the error).  Same would be true of most anything else currently centered.

> * If parent is specified and mapped, the query widget is centered at the center of parent.

I am not sure what 'mapped' means.  With patch, SyntaxError continues to  be centered on screen even with editor visible and focused.  Will say more on the PR.

> Its position and size can be corrected so that it fits in the virtual root window.
I don't understand this.  Will check the code.