In fact, you'll notice that the 8 sizing boxes have disappeared from your window borders. They have "moved" to the control you just added, because now, the control is selected and the window is no longer selected. So, in addition to letting you size a control or your window, the sizing boxes also show you which control (or your window) is selected.
Also, the 4 numbers in the Status bar change to show the selected control's X and Y position (within your window, where a zero X and zero Y position would be at the upper left corner), and the control's width and height. These values are in Dialog Units (DLU).
If you have numerous controls in your window, you can select any particular control just by moving the mouse pointer over that control, and clicking the left mouse button.
Just as you used the sizing boxes to size your window, you can now size the control with those same sizing boxes.
Experiment with sizing the slider you added to the window, to make it bigger.
Move a control
You can also move the control to a new position inside your window. To do so, move the mouse pointer over the control itself (ie, not one of the sizing boxes). Press and hold down the left mouse button. Do not release the button. You should see the mouse pointer change to a four-headed arrow as so:
While still holding down the left mouse button, move the mouse around your window. You'll see that a box (which is about the size of the control) follows the mouse.
When you move that box to the position where you want the control, simply release the mouse button to "drop" that control at the new location. The mouse pointer will return to normal, and you'll see the control at its new position.
Alternately, if a control is already selected, you can move it by using the arrow keys on your keyboard. This is useful for making fine, incremental adjustments to a control's position.
Note: When you add a control to a window or position it, its final placement may be determined by guides or margins, or whether you have the layout grid turned on.
Select several controls
You can select several controls simultaneously, for example, if you want to simultaneously move them all to a new position. You can select several controls in a couple different ways.
First, if the controls are physically next to each other, you can "lasso" them all by dragging a box around the entire group of controls. Start by moving the mouse to a blank area above, and to the left of, the controls you want to select. Press and hold down the left mouse button. You should see the mouse cursor change to a hollow box with a crosshair inside, as so:
While still holding down the left mouse button, move the mouse down, and to the left. You'll see that you can draw a box that surrounds adjacent controls. In the picture below, the box has been drawn around both a slider, and a button control:
When the box surrounds all of the controls you wish to select, then release the mouse button. You should now see sizing boxes along the edges of each of those controls. They are all simultaneously selected.
Once the controls are simultaneously selected, you can move the mouse over any one of them. Press and hold down the SHIFT key on your keyboard. Now, also press and hold down the left mouse button. You'll see the cursor change to a 4-headed arrow and you can now drag all those controls simultaneously to a new position. (You can also use the arrow keys on your keyboard to move the group of controls).
Another way to simultaneously select controls is to first press and hold down the CTRL key on your keyboard. Now move the mouse over some control and click the left mouse button. That will select this one control. Without releasing the CTRL key, move the mouse pointer over another control, and click the left mouse button. That will select this other control as well. Repeat this until you have selected all the desired controls. This is how you can simultaneously select numerous controls that are not adjacent.
Now you can hold down the SHIFT key and click on some control to move them all.
The dominant control
When you select several controls, one (and only one) of them is the "dominant" control. Its sizing boxes are solid squares, whereas the sizing boxes on the other selected controls are hollow boxes. As you'll discover later, the dominant control can sometimes determine what effect some of the Toolbox's operation buttons have.
You can change which one of the selected controls is the dominant one by holding down the SHIFT key on your keyboard, and clicking the mouse on that control. You'll see its sizing boxes turn solid, whereas the other controls will have hollow boxes. In the picture below, both the slider and push button are selected. The slider is the dominant control.
The 4 numbers in the Status bar show the Dominant control's X and Y position, and width and height.