sl0ptty: focus follows the mouse, with the guards that make it liveable

Hovering a pane focuses it. The feature is three lines; the reason it is not
obnoxious is the guards, and each one is a test:

  - not while the prefix is held, so a chord cannot be hijacked by a nudge of
    the mouse;
  - not through the finder, whose rows are on top of the hit list anyway;
  - not during a drag, which owns the mouse until the button comes up;
  - not onto a collapsed pane. Only 'pane:' and 'title:' targets count, never
    the frame rect's 'focus:', so the pointer crossing a stack of headers does
    not expand three panes on its way past.

Off with 'focus_follows_mouse false'; clicking still focuses either way.

This needed the client to ask for ?1003 (any motion) rather than ?1002 (motion
while a button is held), which fixes something that was quietly broken: a pane
requesting any-event tracking could never receive hover, because the outer
terminal was never asked to report it. It can now, and a pane that asked only
for clicks still gets none — both tested.

One real bug behind that: our decoder represents 'no button held' as
MBTN_UNKNOWN, and pane_send_mouse was passing it to the encoder as a concrete
button. lib-vt has ghostty_mouse_event_clear_button() for exactly this, and
without it bare motion encoded to nothing at all.

320 checks green.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/pane.c b/src/pane.c
index ac03134..38b8f41 100644
--- a/src/pane.c
+++ b/src/pane.c
@@ -339,7 +339,10 @@ void pane_send_mouse(pane_t *p, const input_event_t *ev) {
 
   GhosttyMousePosition pos = {.x = (float)ev->mx, .y = (float)ev->my};
   ghostty_mouse_event_set_position(p->mev, pos);
-  ghostty_mouse_event_set_button(p->mev, (GhosttyMouseButton)ev->button);
+  /* "no button held" is its own thing, not button zero: bare motion (hover)
+   * must be encoded as none, or a pane in any-event tracking sees nothing. */
+  if (ev->button == MBTN_UNKNOWN) ghostty_mouse_event_clear_button(p->mev);
+  else ghostty_mouse_event_set_button(p->mev, (GhosttyMouseButton)ev->button);
   ghostty_mouse_event_set_action(p->mev, (GhosttyMouseAction)ev->maction);
   ghostty_mouse_event_set_mods(p->mev, (GhosttyMods)ev->mods);