Bug 1947508 - Give some slack to the caret blink timer. r=dholbert

Something like this perhaps? Given the usual caret blink rate this
should be enough. We can reduce the slack if needed.

Differential Revision: https://phabricator.services.mozilla.com/D237768

layout/base/nsCaret.cpp

@@ -601,6 +601,11 @@ nsCaret::NotifySelectionChanged(Document*, Selection* aDomSel, int16_t aReason,
 }
 
 void nsCaret::ResetBlinking() {
+  // How many milliseconds we allow the timer to get off-sync when resetting
+  // blinking too often. 50 is not likely to be user observable in practice,
+  // it's ~4 animation frames at 60fps.
+  static const auto kBlinkTimerSlack = TimeDuration::FromMilliseconds(50);
+
   mIsBlinkOn = true;
 
   if (mReadOnly || !IsVisible()) {
@@ -608,17 +613,28 @@ void nsCaret::ResetBlinking() {
     return;
   }
 
-  const auto blinkTime = LookAndFeel::CaretBlinkTime();
-  if (blinkTime <= 0) {
+  const int32_t oldBlinkTime = mBlinkTime;
+  mBlinkTime = LookAndFeel::CaretBlinkTime();
+  if (mBlinkTime <= 0) {
     StopBlinking();
     return;
   }
 
   mBlinkCount = LookAndFeel::CaretBlinkCount();
+
+  const auto now = TimeStamp::NowLoRes();
+  const bool mustResetTimer = mBlinkTime != oldBlinkTime ||
+                              mLastBlinkTimerReset.IsNull() ||
+                              (now - mLastBlinkTimerReset) > kBlinkTimerSlack;
+  if (!mustResetTimer) {
+    return;
+  }
+
   if (!mBlinkTimer) {
     mBlinkTimer = NS_NewTimer();
   }
-  mBlinkTimer->InitWithNamedFuncCallback(CaretBlinkCallback, this, blinkTime,
+  mLastBlinkTimerReset = now;
+  mBlinkTimer->InitWithNamedFuncCallback(CaretBlinkCallback, this, mBlinkTime,
                                          nsITimer::TYPE_REPEATING_SLACK,
                                          "CaretBlinkCallback");
 }
@@ -626,6 +642,7 @@ void nsCaret::ResetBlinking() {
 void nsCaret::StopBlinking() {
   if (mBlinkTimer) {
     mBlinkTimer->Cancel();
+    mLastBlinkTimerReset = TimeStamp();
   }
 }
 
@@ -696,6 +713,7 @@ void nsCaret::CaretBlinkCallback(nsITimer* aTimer, void* aClosure) {
   if (!theCaret) {
     return;
   }
+  theCaret->mLastBlinkTimerReset = TimeStamp();
   theCaret->mIsBlinkOn = !theCaret->mIsBlinkOn;
   theCaret->SchedulePaint();
 

layout/base/nsCaret.h

@@ -202,6 +202,11 @@ class nsCaret final : public nsISelectionListener {
   mozilla::WeakPtr<mozilla::dom::Selection> mDomSelectionWeak;
 
   nsCOMPtr<nsITimer> mBlinkTimer;
+  // Last time we reset the blink timer. We give it some slack to avoid
+  // resetting it too often. This gets cleared when CaretBlinkCallback fires,
+  // because the point of this variable is just to avoid resetting too many
+  // times in a single blink cycle.
+  mozilla::TimeStamp mLastBlinkTimerReset;
 
   CaretPosition mCaretPosition;
 
@@ -214,6 +219,11 @@ class nsCaret final : public nsISelectionListener {
    * blinking.
    */
   int32_t mBlinkCount = -1;
+  /**
+   * Current blink time (the value that LookAndFeel::CaretBlinkTime() gave us
+   * when we most recently reset our blinking).
+   */
+  int32_t mBlinkTime = -1;
   /**
    * mHideCount is not 0, it means that somebody doesn't want the caret
    * to be visible.  See AddForceHide() and RemoveForceHide().