In a native terminal running a TUI with mouse tracking on (claude, codex), Shift
is the "let me select text" modifier: it bypasses the application's mouse
reporting so the emulator selects locally. Users bring that habit here, where it
did nothing — measured, `hasSelection` was already false during a Shift+drag and
no clearSelection call ran at all, because there was never a selection to clear.
The mismatch is that the two Shifts mean different things. xterm reads Shift as
"force selection", but that path is only taken when the application really has
mouse tracking on. The server strips the mouse DECSETs for claude/codex/gemini
(isAltScreenStripMode), so xterm's mouseTrackingMode is permanently `none`, that
branch is unreachable, and Shift instead lands in _onIncrementalClick — which
EXTENDS an existing selection. Extension is a no-op while selectionStart is
empty, so the drag had no anchor.
So plant the anchor xterm is missing. The listener sits on the capture phase of
the `.xterm` root, an ancestor of the `.xterm-screen` that SelectionService binds
to, and therefore runs before xterm's own mousedown; xterm then extends from our
anchor and the drag behaves like any other. Length is 0 so a Shift+click without
a drag does not select a stray character. An existing selection is left alone —
that is a genuine extend gesture, and xterm handles it correctly.
Right-click copies the selection (the mintty/PuTTY convention), completing the
gesture: until now there was nowhere for a finished selection to go. With no
selection the native menu is not hijacked — taking it away while offering
nothing in return is a pure loss.
(cherry picked from commit 7ab5015737)
