…ition

Introduce a per-window list of damaged rectangles (struct redraw_damage)
and redraw_damage_window() to record them, redraw_client_damage() to
consume them by composing exactly the damaged cells (via a new
redraw_draw_damage_rect(), which also force-refreshes any pane-status
span it touches, since window_make_pane_status()'s content-diff check
has no way to know the physical cells were disturbed by something else).

redraw_draw_span() now takes an explicit [clip_x, clip_x + clip_n) range
instead of always drawing a span's full width, so a damage rectangle can
redraw just the portion of a span it actually covers.

screen_write_redraw_cb() - the fallback when a write can't be applied
directly to the terminal - now reports damage for just the affected row
via this mechanism, instead of unconditionally flagging the whole pane
for a full redraw.

window_pane_redraw_floating() reports damage for only a floating pane's
old and new rectangle (via redraw_damage_window(), grown by one cell to
cover its border frame - see the "floating" case in screen-redraw.c),
instead of the caller falling back to a full window redraw. Any other
pane whose *scrollbar strip* - not its whole body - intersects either
rectangle still gets PANE_REDRAWSCROLLBAR directly, since scrollbars
aren't covered by the damage system.

redraw_client_damage() (added previously, unused until now) is called
from server_client_check_redraw()'s normal redraw pass, and
server_client_any_pane_redraw() now also checks for pending window
damage so a client with only damage (no PANE_REDRAW/PANE_REDRAWSCROLLBAR
flags) still gets its redraw pass run.

server_client_check_redraw() now returns whether the redraw was deferred
(waiting for outstanding tty output to drain) rather than performed. A
deferred redraw no longer escalates to a full CLIENT_REDRAWWINDOW to
avoid losing what was pending - server_client_loop() now only clears
PANE_REDRAW, PANE_REDRAWSCROLLBAR and window damage once every client
viewing a window actually drew this pass (tracked via a new per-window
redraw_deferred flag), otherwise they're left in place and retried in
their normal, narrowly-scoped form.

server_client_set_session() now redraws only if the client's session or
current window actually changed, not on every call (e.g. switch-client
-t= from clicking a pane name in the status line resolves here even
when nothing besides the active pane changed).

A drag callback's mouse_drag_update() now opens a sync region itself
(tty_sync_start()) before its first write, so a fast-path write it makes
directly and a later correction arriving via redraw_client_damage() end
up in the same atomic terminal update instead of two visible frames.

window_set_active_pane() unconditionally called server_redraw_window(w)
on every active-pane change, redrawing every pane's content even though
only the previous and new active pane's border/status appearance
actually changed. Unzooming (which does change every pane's geometry)
still gets the full redraw; otherwise this now only redraws borders and
status.

…pane

redraw instead of full client redraw

The three interactive mouse-drag paths that move or resize a floating
pane (move-pane -M's Alt-drag, split-window/new-pane's interactive
resize, and resize-pane's own border drag) each unconditionally called
server_redraw_window(w), redrawing every pane in the window for a change
that only ever disturbs the floating pane's own old and new rectangle.
Switch all three to window_pane_redraw_floating().

…redraw

popup_handle_drag()'s MOVE and SIZE branches each called
server_redraw_client(c) unconditionally, redrawing the client's entire
window on every drag step. Report damage for just the popup's old and
new rectangle instead, via a new popup_damage() (translating from raw
client/tty coordinates into window coordinates), and set
CLIENT_REDRAWOVERLAY so the popup itself still redraws.

…dex to catch an unserviced edge-case.

From github.com/darlingm/tmux, branch pr5516-regression-fixes. Adds 9
regression tests covering gaps found in the redraw-damage-rectangles
branch: screen-write full/region redraw fallback, same-session window
switches, wide-character clipping at damage edges, pane prompts and
status lines surviving damage, floating-pane status format refresh,
and multi-client damage delivery.

redraw-multiclient.sh is adapted here to use ASCII pane borders
(pane-border-lines simple) instead of darlingm's original UTF-8
borders: the original reliably "failed" under this test's nested
tmux-in-tmux harness (relaying through an outer tmux client) due to
that harness mis-rendering a cell that held a multi-byte UTF-8 border
character being overwritten by later plain content - confirmed to be
a nested-relay artifact, not a real bug, by replaying the identical
drag sequence against a real terminal (xterm), where it never
reproduces. ASCII borders avoid the artifact; the test still reliably
catches the real "damage consumed by only one client" bug it targets
(verified by reintroducing that bug and confirming the test fails).

The other 8 tests are added verbatim from darlingm's branch. Four of
them (popup-drag-status-line.sh, popup-drag-wide-character.sh,
popup-drag-pane-prompt.sh, switch-client-redraw.sh) currently FAIL on
this branch, since the source fixes they test for have not been
merged yet - only the tests are being added here.

Co-Authored-By: Michael K. Darling <darlingm@gmail.com>
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Fixes the four bugs caught by the regression tests added in aed1209
(popup-drag-status-line.sh, popup-drag-wide-character.sh,
popup-drag-pane-prompt.sh, switch-client-redraw.sh), based on fixes
from Michael K. Darling (github.com/darlingm/tmux, pr5516-regression-
fixes), reviewed and adapted:

- server-client.c: server_client_set_session()'s check for whether the
  client's window actually changed compared old->curw to s->curw, but
  when old == s these read the same, already-updated field, so a
  same-session window switch was never detected. Compare against the
  client's own cached redraw scene instead (redraw_client_has_window(),
  new in screen-redraw.c/tmux.h). Taken from darlingm as-is.

- popup.c: popup_damage() only translated a popup's client-coordinate
  rectangle into window coordinates, so a popup dragged across the
  status line never triggered a status-line redraw once it moved away -
  status_redraw()'s own "skip if content unchanged" optimization
  suppressed it, since only the popup moved, not the status content.
  Now detects overlap with the status line and forces a redraw via the
  existing (previously unused) CLIENT_REDRAWSTATUSALWAYS flag, and
  properly clips the reported rectangle to the pane area for
  status-at-top/bottom/off. Taken from darlingm as-is.

- screen-redraw.c: redraw_draw_damage_rect() clipped a span to a damage
  rectangle's raw geometric edges, which have no idea what's in the
  grid, so a clip edge could land mid-character and tear a wide
  character in half. Added redraw_damage_grow_span_clip(): widen the
  clip by one cell on each edge that isn't already at the span's own
  boundary. Reimplemented simpler than darlingm's version (which walked
  grid cells per span type via a switch and direct grid lookups) -
  since no grid cell is ever wider than two columns, an unconditional
  one-cell margin is always enough to pull a split character back in,
  with no need to inspect grid content at all.

- screen-redraw.c: redraw_draw_damage_rect() also never re-overlaid a
  pane's active in-pane prompt after drawing its underlying content, so
  damage crossing a prompt row erased it until an unrelated redraw
  restored it. Factored the existing full-redraw prompt-building code
  into a shared redraw_make_pane_prompt() helper and added
  redraw_damage_draw_pane_prompt(), which recomposes the prompt over
  the drawn range. Taken from darlingm as-is.

All 9 regression tests in regress/ now pass. redraw_damage_grow_span_clip
was verified independently by disabling it and confirming
popup-drag-wide-character.sh reproduces its original failure.

Co-Authored-By: Michael K. Darling <darlingm@gmail.com>
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>