WebUIControl.LockVertical
From Xojo Documentation
Read-Only Property (As Boolean )
LockVertical overrides LockTop and LockBottom. It allows you to proportionally lock a control's position to keep it centered within the parent control or web page.
Notes
For example, if you place a control in the center of the page, and sets both LockHorizontal and LockVertical, the control will stay in the center of the page.