always refetch when a date nav invalidates an in-progress event sourc…

…e fetch

packages/__tests__/src/event-source/navigation.js

@@ -0,0 +1,45 @@
+
+describe('event fetching while date-navigating', function() {
+
+  // https://github.com/fullcalendar/fullcalendar/issues/4975
+  it('renders events when doing next() and then prev()', function(done) {
+
+    initCalendar({
+      defaultView: 'dayGridMonth',
+      defaultDate: '2020-02-11',
+      events: function(arg, callback) {
+        if (arg.startStr.indexOf('2020-01-26') === 0) { // for Feb
+          setTimeout(function() {
+            callback([
+              { start: '2020-02-15' } // middle of month
+            ])
+          }, 100)
+        } else if (arg.startStr.indexOf('2020-03-01') === 0) { // for March
+          setTimeout(function() {
+            callback([
+              { start: '2020-03-15' } // middle of month
+            ])
+          }, 100)
+        } else {
+          throw new Error('bad range')
+        }
+      }
+    })
+
+    setTimeout(function() {
+
+      currentCalendar.next()
+      setTimeout(function() {
+
+        currentCalendar.prev()
+        setTimeout(function() {
+
+          expect($('.fc-event').length).toBe(1)
+          done()
+
+        }, 1000) // after everything
+      }, 50) // before second fetch finishes
+    }, 200) // let first fetch finish
+  })
+
+})

packages/core/src/reducers/eventSources.ts

@@ -92,6 +92,7 @@ function isSourceDirty(eventSource: EventSource, fetchRange: DateRange, calendar
   } else {
     return !calendar.opt('lazyFetching') ||
       !eventSource.fetchRange ||
+      eventSource.isFetching || // always cancel outdated in-progress fetches
       fetchRange.start < eventSource.fetchRange.start ||
       fetchRange.end > eventSource.fetchRange.end
   }