Docking layout depends on the order of sibling controls. Controls are docked "button up", so the last control in the collection is docked first. A docked control only take the layout of previously docked siblings into account. Hence the control with Dock=Fill should be first (top) in the sibling order, if you want it to take the other docked controls into account. If it is not the first control, earlier controls will overlap it.
This can be confusing because the sibling-order is not necessarily the same as the visual order, and the sibling order is not always apparent from the design view.
The Document outline window (View -> Other Windows -> Document outline) gives a useful tree-view over the control hierarchy and order, and allows you to change the sibling order of controls.
You can also change sibling order directly in the designer by context menu -> Bring to front / Send to back, which moves the control to be first or last of the siblings. These menu labels may be somewhat confusing since the actual effect depends on the layout model.
With fixed positioned controls, the 2D position is independent of the sibling order, but when controls are overlapping, the control earliest in the order will be "on top", hiding part of siblings later in the order. In this context Bring to front / Send to back makes sense.
Inside flow- or table-layout panels, the creation order determines the visual order of the controls. There is no overlapping controls. So bring to front/send to back really means make first or last in the order of controls.
With docked layout, the bring to front / send to back may be even more confusing since it determines in which order the docking is calculated, so "bring to front" on a fill-docked control will place the control in the middle of the parent, taking all edge-docked controls into account.
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