Keep the BalloonTip open when the BalloonTip's arrow prevents the cursor
from being inside the spawning ToolTipWidget, which triggers the
ToolTipWidget's leaveEvent and would previously close the BalloonTip.
When that happens track the cursor until it either leaves the
ToolTipWidget's bounding box or leaves the BalloonTip and goes back to
the ToolTipWidget, and respectively close the BalloonTip or leave it
open.
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