Use Qukeys min prior interval only after printable characters

Some users have pointed out that certain keys (in particular, `space` &
`backspace`) are inconvenient to require the minimum prior interval to make a
qukey resolve to a modifier (especially `shift`). We could blacklist those keys,
but it's hard to predict what they all might be. The problem is mainly one for
very fast typists, and therefore I expect it to show up when following the
"normal" printable keys, not often other keys.

This could also be made into a configurable list, but I'd prefer not to do so
unless there's serious demand for it, as Qukeys already has too many settings.

Signed-off-by: Michael Richters <gedankenexperimenter@gmail.com>

docs/NEWS.md

@@ -20,8 +20,9 @@ the output, particularly when typing fast:
 - `Qukeys.setMinimumHoldTime(ms)` sets the minimum duration of a qukey press
   required for it to be eligible to take on its alternate (modifier) value.
 - `Qukeys.setMinimumPriorInterval(ms)` sets the minimum interval between the
-  previous non-modifier key press and the press of the qukey required to make
+  previous printable (letters, numbers, and punctuation) key press and the press
-  the qukey eligible to take on its alternate (modifier) value.
+  of the qukey required to make the qukey eligible to take on its alternate
+  (modifier) value.
 
 ### KALEIDOSCOPE_API_VERSION bump
 

src/kaleidoscope/plugin/Qukeys.cpp

@@ -185,7 +185,7 @@ bool Qukeys::processQueue() {
   // key, so we don't need to do it repeatedly later.
   bool qukey_is_spacecadet = isModifierKey(queue_head_.primary_key);
 
-  // If the qukey press followed a non-modifier key too closely, it's not
+  // If the qukey press is followed a printable key too closely, it's not
   // eligible to take on its alternate value unless it's a SpaceCadet-type key.
   if (!Runtime.hasTimeExpired(prior_keypress_timestamp_,
                               minimum_prior_interval_) &&
@@ -291,8 +291,13 @@ void Qukeys::flushEvent(Key event_key) {
   KeyAddr queue_head_addr = event_queue_.addr(0);
   uint8_t keyswitch_state = event_queue_.isRelease(0) ? WAS_PRESSED : IS_PRESSED;
 
-  // If the flushed event is a keypress of a non-modifier, record its timestamp:
+  // If the flushed event is a keypress of a printable symbol, record its
-  if (!event_queue_.isRelease(0) && !isModifierKey(event_key)) {
+  // timestamp. This lets us suppress some unintended alternate values seen by
+  // fast typists by requiring a minimum interval between this keypress and the
+  // next qukey press in order for that qukey to become alternate-eligible.
+  if (!event_queue_.isRelease(0) &&
+      ((event_key >= Key_A && event_key <= Key_0) ||
+       (event_key >= Key_Minus && event_key <= Key_Slash))) {
     prior_keypress_timestamp_ = event_queue_.timestamp(0);
   }