Improve Stopwatch behavior (#18930)

- Don't send empty stopwatch over and over again, only send once.
- Stop interval to update stopwatch's timer when there is no more stopwatch.

routers/web/repo/issue_stopwatch.go

@@ -9,7 +9,9 @@ import (
 	"strings"
 
 	"code.gitea.io/gitea/models"
+	"code.gitea.io/gitea/models/db"
 	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/eventsource"
 )
 
 // IssueStopwatch creates or stops a stopwatch for the given issue.
@@ -59,6 +61,18 @@ func CancelStopwatch(c *context.Context) {
 		return
 	}
 
+	stopwatches, err := models.GetUserStopwatches(c.Doer.ID, db.ListOptions{})
+	if err != nil {
+		c.ServerError("GetUserStopwatches", err)
+		return
+	}
+	if len(stopwatches) == 0 {
+		eventsource.GetManager().SendMessage(c.Doer.ID, &eventsource.Event{
+			Name: "stopwatches",
+			Data: "{}",
+		})
+	}
+
 	url := issue.HTMLURL()
 	c.Redirect(url, http.StatusSeeOther)
 }