Grab sometimes wants a viewable window

# This program brings up two toplevels. The Quit button on top2 does not work
# because of the grab on .tree1. After bringing up the combobox list and then
# clicking outside the list to hide it again the Quit button of top2 becomes
# active. This happens because the [grab .tree1] command performed by the
# combobox bindings fails ("grab failed: window not viewable"). However, the
# same command executed by pressing F2 has no problems to set the grab on
# .tree1, even though it is still not viewable. Pressing F1 will show which
# widget currently has the grab, if any.

wm withdraw .
frame .tree1

toplevel .tree1.top
ttk::combobox .tree1.top.box -state readonly \
  -values {"Option 1" "Option 2" "Option 3"}
label .tree1.top.grab -textvariable grab
button .tree1.top.quit -text Quit -command exit

grid .tree1.top.box -padx 8 -pady 8 -sticky nwe
grid .tree1.top.grab -padx 8 -pady 8
grid .tree1.top.quit -padx 8 -pady 8

toplevel .top2
button .top2.quit -text Quit -command exit
grid .top2.quit -padx 8 -pady 8

grab .tree1

bind all <F1> {set grab [grab current]}
bind all <F2> {grab .tree1}