Closes#9.
It was clamped against a GUESSED height — 230 px with the upload submenu, 110
without. The menu is far taller than that with a selection, so right-clicking on
a row near the bottom hid half the entries, delete among them.
It is measured now: placed above the cursor when it does not fit below, pinned
to the top with its own scrollbar when it fits neither way. Its height depends
on which actions the caller passes, so no constant could have stayed right.
rouggy
e4217da010