Show filtered lists and their shared focus

A list may now be filtered (each t in tasks where !t.done), and several
filtered lists over one list share one focus: the focus view names an
item, so Vega asks Pure which row shows it and highlights it only in the
list that holds it; the other lists have no focus (null in the web
surface's JSON, and the spoken surface names the item where it is). A
click or point on a row points at the item that row shows, and the wheel
on a list without the focus focuses the row under it. Pure's rows now
take a mutable app.

examples/today.aup shows what is open and what is done; its CLI output
matches a model over random adds, moves, toggles and points into either
list, and the web surface leaves the other list without a focus.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/main.c b/src/main.c
index 46b672f..b9ea91e 100644
--- a/src/main.c
+++ b/src/main.c
@@ -95,8 +95,10 @@ static bool mouse(unsigned button, unsigned line) {
     const VegaPart *part = &screen.parts[index];
     if (button == 64 || button == 65) {
         size_t focus = vega_focus(&app, part), rows = vega_rows(&app, part);
-        if (button == 64 ? !focus : focus + 1 >= rows) return false;
-        item = button == 64 ? focus - 1 : focus + 1;
+        if (focus < rows) { /* else the wheel focuses the row under it */
+            if (button == 64 ? !focus : focus + 1 >= rows) return false;
+            item = button == 64 ? focus - 1 : focus + 1;
+        }
     } else if (button) return false;
     return vega_point(&app, part, item);
 }