Treat key combinations as a unit (combos)

The display now only updates on key-down (the combo grows as you press)
and when the last key is released (the empty payload starts the linger
with the last full combo). Intermediate releases no longer shrink the
display, so the order you let go of a shortcut doesn't matter: Ctrl+Shift+N
stays Ctrl+Shift+N whether Ctrl, Shift, or N goes up first, and the 1s
linger shows the full combo instead of a partial release state. Matches
the keybinding-tutorial use case better than keyviz, which shrinks
key-by-key on spaced releases.

key-visualizer.lua

@@ -145,6 +145,13 @@ local function emit()
   end
 end
 
+-- Combos: a combination of keys is treated as a unit. The display only
+-- updates on key-down (the combo grows as you press) and when the last key
+-- is released (the empty payload starts the panel's linger with the last
+-- full combo). Intermediate releases never shrink the display, so the order
+-- in which you let go of a shortcut doesn't matter: Ctrl+Shift+N stays
+-- Ctrl+Shift+N whether you release Ctrl, Shift, or N first.
+--
 -- state: 0 = released, 1 = pressed, 2 = repeat (ignored).
 hl.on("input.keyboard.key", function(keycode, timeMs, state)
   if state == 2 then return end
@@ -158,6 +165,7 @@ hl.on("input.keyboard.key", function(keycode, timeMs, state)
       end
       if not found then combo[#combo + 1] = keycode end
     end
+    emit()
   else
     pressed[keycode] = false
     if not MODS[keycode] then
@@ -168,7 +176,12 @@ hl.on("input.keyboard.key", function(keycode, timeMs, state)
         end
       end
     end
+    -- Only emit when the last key goes up: intermediate releases keep the
+    -- full combo on screen (the panel lingers it after the empty payload).
+    local any_down = false
+    for _, down in pairs(pressed) do
+      if down then any_down = true break end
+    end
+    if not any_down then emit() end
   end
-
-  emit()
 end)