Hello!
Here's how to replicate:
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Load the time span demo
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From the console, add a timespan early in the Gantt so you see a timespan appear:
gantt.highlightTimeSpans([{name: 'test', startDate: new Date(2019, 0, 1), endDate: new Date(2019, 0, 3) }])
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Right click on the timeline, date range
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Click on the right arrow until the timespan is no longer visible
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Click on the left arrow until the timespan comes back into view
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The timespan is not visible anymore.
A workaround is to call highlightTimeSpans() every time the timeline date range changes. A bit inefficient though.