Bug 1660933 - Update default mousewheel animation durations. r=jaws

Also detect existing users and carry forward their customizations if needed.
If no customization was done, we set a pref to indicate they should remain
at their old defaults, which we can gradually migrate to the new values.

Differential Revision: https://phabricator.services.mozilla.com/D88162

browser/components/BrowserGlue.jsm

@@ -3202,7 +3202,7 @@ BrowserGlue.prototype = {
   _migrateUI: function BG__migrateUI() {
     // Use an increasing number to keep track of the current migration state.
     // Completely unrelated to the current Firefox release number.
-    const UI_VERSION = 96;
+    const UI_VERSION = 97;
     const BROWSER_DOCURL = AppConstants.BROWSER_CHROME_URL;
 
     if (!Services.prefs.prefHasUserValue("browser.migration.version")) {
@@ -3790,6 +3790,46 @@ BrowserGlue.prototype = {
       Services.prefs.clearUserPref(oldPrefName);
     }
 
+    if (currentUIVersion < 97) {
+      let userCustomizedWheelMax = Services.prefs.prefHasUserValue(
+        "general.smoothScroll.mouseWheel.durationMaxMS"
+      );
+      let userCustomizedWheelMin = Services.prefs.prefHasUserValue(
+        "general.smoothScroll.mouseWheel.durationMinMS"
+      );
+
+      if (!userCustomizedWheelMin && !userCustomizedWheelMax) {
+        // If the user has an existing profile but hasn't customized the wheel
+        // animation duration, indicate that they need to be migrated to the new
+        // values by setting their "migration complete" percentage to 0.
+        Services.prefs.setIntPref(
+          "general.smoothScroll.mouseWheel.migrationPercent",
+          0
+        );
+      } else if (userCustomizedWheelMin && !userCustomizedWheelMax) {
+        // If they customized just one of the two, save the old value for the
+        // other one as well, because the two values go hand-in-hand and we
+        // don't want to move just one to a new value and leave the other one
+        // at a customized value. In both of these cases, we leave the "migration
+        // complete" percentage at 100, because they have customized this and
+        // don't need any further migration.
+        Services.prefs.setIntPref(
+          "general.smoothScroll.mouseWheel.durationMaxMS",
+          400
+        );
+      } else if (!userCustomizedWheelMin && userCustomizedWheelMax) {
+        // Same as above case, but for the other pref.
+        Services.prefs.setIntPref(
+          "general.smoothScroll.mouseWheel.durationMinMS",
+          200
+        );
+      } else {
+        // The last remaining case is if they customized both values, in which
+        // case also we leave the "migration complete" percentage at 100, as no
+        // further migration is needed.
+      }
+    }
+
     // Update the migration version.
     Services.prefs.setIntPref("browser.migration.version", UI_VERSION);
   },

modules/libpref/init/StaticPrefList.yaml

@@ -3769,12 +3769,17 @@
 
 - name: general.smoothScroll.mouseWheel.durationMaxMS
   type: RelaxedAtomicInt32
-  value: 400
+  value: 200
   mirror: always
 
 - name: general.smoothScroll.mouseWheel.durationMinMS
   type: RelaxedAtomicInt32
-  value: 200
+  value: 50
+  mirror: always
+
+- name: general.smoothScroll.mouseWheel.migrationPercent
+  type: RelaxedAtomicInt32
+  value: 100
   mirror: always
 
 - name: general.smoothScroll.other.durationMaxMS