Disable stylus barrel button for a brief period after pen up.

This should prevent metalayer triggering when a user is
writing/sketching while accidentally holding down the stylus button.

Bug: 767440
Test: ash_unittests --gtest_filter=PaletteTrayTest*
Change-Id: I98a1dc4be2bb888c6cc06e3e93a60b73b56c6a8d
Reviewed-on: https://chromium-review.googlesource.com/683254
Commit-Queue: Vladislav Kaznacheev <kaznacheev@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504419}

ash/system/palette/tools/metalayer_mode.cc

@@ -28,6 +28,10 @@ namespace {
 const char kToastId[] = "palette_metalayer_mode";
 const int kToastDurationMs = 2500;
 
+// If the last stroke happened within this amount of time,
+// assume writing/sketching usage.
+const int kMaxStrokeGapWhenWritingMs = 1000;
+
 }  // namespace
 
 MetalayerMode::MetalayerMode(Delegate* delegate)
@@ -92,9 +96,21 @@ void MetalayerMode::OnTouchEvent(ui::TouchEvent* event) {
       ui::EventPointerType::POINTER_TYPE_PEN)
     return;
 
+  if (event->type() == ui::ET_TOUCH_RELEASED) {
+    previous_stroke_end_ = event->time_stamp();
+    return;
+  }
+
   if (event->type() != ui::ET_TOUCH_PRESSED)
     return;
 
+  if (event->time_stamp() - previous_stroke_end_ <
+      base::TimeDelta::FromMilliseconds(kMaxStrokeGapWhenWritingMs)) {
+    // The press is happening too soon after the release, the user is most
+    // likely writing/sketching and does not want the metalayer to activate.
+    return;
+  }
+
   // The stylus "barrel" button press is encoded as ui::EF_LEFT_MOUSE_BUTTON
   if (!(event->flags() & ui::EF_LEFT_MOUSE_BUTTON))
     return;

ash/system/palette/tools/metalayer_mode.h

@@ -83,6 +83,8 @@ class ASH_EXPORT MetalayerMode : public CommonPaletteTool,
 
   bool voice_interaction_context_enabled_ = false;
 
+  base::TimeTicks previous_stroke_end_;
+
   base::WeakPtrFactory<MetalayerMode> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(MetalayerMode);