`PointerRoute::UnusedButton`'s own doc named this row: "Stage 1b's B4
gives the middle button a meaning (PRIMARY-selection paste on Linux) and
lands here." B4 splits that variant, as §2a said it would.
A middle PRESS is now `PointerRoute::MiddlePress` and reads the PRIMARY
selection, shipping it as the same `Paste` wire operation Ctrl-V uses.
Its RELEASE stays unused, like the right button's --- the paste happens
once, on the press.
PRIMARY and CLIPBOARD are different selections with different contents:
the clipboard holds what was last explicitly copied, PRIMARY holds what
is currently selected. Reading the wrong one still produces a paste,
just not the one the platform convention promises, so the row asserts
the SOURCE rather than that a paste happened.
`middle_click_paste_source` is the seam that makes that assertable
without an OS clipboard; `read_os_selection` takes the source and uses
arboard's `GetExtLinux` for PRIMARY.
Two rows, three mutations, each firing:
source = Clipboard -> the source row
middle press unrouted -> the routing row
release also pastes -> the routing row
Three existing rows encoded the old behaviour --- that a middle press is
semantics-free. They are updated to keep testing what they SAY rather
than being weakened to accommodate B4: the routing row now covers
Back/Forward/Other plus the middle RELEASE, and the two effect/order
rows switch to `Back`, a button that still has no semantics. Widening
them to accept the new meaning would have left no row asserting that
semantics-free buttons stay inert.
Levi Neuwirth
493db8f965