> Please note that if there is no suitable window with at least 10 lines high,
 > it still displays the *Backtrace* buffer in random windows, every time
 > in another window because it doesn't remember a previous window.

For starters, users can customize the number of lines by supplying a
'window-min-height' entry.  They can also provide an additional action
function like 'display-buffer-below-selected' as fallback.  And
finally we could try what I suggested earlier: Fill the *Backtrace*
buffer first and pop to it afterwards using 'fit-window-to-buffer'.
The attached untested patch tries to do that.

martin