flush the cache on changes of enableColors

src/_locales/en/messages.json

@@ -181,7 +181,7 @@
     "message": "Force use of plain text in event descriptions"
   },
   "settings.enableColors": {
-    "message": "Enable support for event colors (experimental; calendars must be unsubscribed, Thunderbird restarted (?), and the calendars resubscribed)"
+    "message": "Enable support for event colors (experimental; enabling/disabling reloads all calendars)"
   },
 
   "eventdialog.conferenceLabel": {

src/legacy/modules/gdataUI.jsm

@@ -71,20 +71,26 @@ function register() {
     },
   });
 
-  let { doEnableColors, doDisableColors, getMessenger } = ChromeUtils.import(
+  let { doEnableColors, doDisableColors, flushCache, getMessenger } = ChromeUtils.import(
     "resource://gdata-provider/legacy/modules/gdataUtils.jsm"
   );
 
   let messenger = getMessenger();
   messenger.storage.onChanged.addListener((settings, _target) => {
     if ("settings.enableColors" in settings) {
-      if (settings["settings.enableColors"].newValue) {
+      // check for true and false explicitly, since `undefined` can also happen
+      if (settings["settings.enableColors"].newValue === true) {
         doEnableColors();
-      } else {
+      } else if (settings["settings.enableColors"].newValue === false) {
         doDisableColors();
       }
+      // We must delete the cache and reload all calendars from the source in order
+      // to add/remove the color categories. Restarting Thunderbird is insufficient
+      // because the locally cached calendar entries lack the color information.
+      flushCache();
     }
   });
+
   let enableColors = messenger.gdataSyncPrefs.get("settings.enableColors", false);
   if (enableColors) {
     doEnableColors();

src/legacy/modules/gdataUtils.jsm

@@ -55,6 +55,7 @@ var EXPORTED_SYMBOLS = [
   "spinEventLoop",
   "doEnableColors",
   "doDisableColors",
+  "flushCache",
   "getMessenger",
 ];
 
@@ -1525,6 +1526,18 @@ function doDisableColors() {
   Services.prefs.setStringPref("calendar.categories.names", calendarCategories);
 }
 
+/**
+ * Deletes the local Google calendar cache and reloads all Google calendars from the source.
+ */
+function flushCache() {
+  let cals = cal.manager.wrappedJSObject.getCalendars();
+  for (let calendar of cals) {
+    if (calendar.type == "gdata") {
+      calendar.mUncachedCalendar.resetLog();
+    }
+  }
+}
+
 class SyncPrefs {
   prefs = {};