Redmine

Thank you for reporting this issue.
The problem is that issueTo.position().top returns 0 when issueTo is display: none.
The following diff fixes this by simply not drawing the line when the target bar is not visible.

diff --git a/app/javascript/controllers/gantt/chart_controller.js b/app/javascript/controllers/gantt/chart_controller.js
index d467c558d..70a39195d 100644
--- a/app/javascript/controllers/gantt/chart_controller.js
+++ b/app/javascript/controllers/gantt/chart_controller.js
@@ -190,6 +190,7 @@ export default class extends Controller {
       const issueTo = this.$(`#task-todo-issue-${relation.issue_to}`)

       if (issueFrom.length === 0 || issueTo.length === 0) return
+      if (!issueTo.is(":visible")) return

       const issueHeight = issueFrom.height()
       const issueFromTop = issueFrom.position().top + issueHeight / 2 - this.#drawTop

Committed the fix in r24856. Thank you.

Merged the fix into 7.0-stable branch in r24863.