Knowledge Base

Title: Control Overlaid by Another Control Fails to Refresh if Moved 
Document Number: Q74519           Publ Date: 29-JUL-1991 
Product Name: Microsoft Visual Basic 
Product Version:  1.00 
Operating System: WINDOWS 

 Summary: 

 Visual Basic does not support overlapping controls. Having overlapping
 controls can result in portions of a control not refreshing correctly. 
 If controls are moved over each other, then one or both of the 
 controls may not correctly refresh even when the controls are moved 
 apart. This is known to happen when controls are resized at run time 
 using the Move method or by changing the Height and Width properties 
 as a result of a Form_Resize event. Because controls must be resized 
 one at a time, it is possible that one control will briefly overlap 
 another control during the resize process at run time. The control 
 that was briefly overlapped may not refresh properly. An example of 
 this behavior is given further below. 

 This behavior can be improved by performing the Refresh method 
 (CtrlName.Refresh) on every overlapping control at run time, after an 
 overlapped control has been moved or after a form that contains 
 overlapping controls has been resized. 
 This is not a problem with Visual Basic, but the nature of overlapping 
 controls under Visual Basic. This behavior occurs at run time in the 
 Visual Basic development environment or as an .EXE program. 
 This information applies to the Visual Basic programming system 
 version 1.0 for Windows. 

 More Information:

 For more information about Visual Basic and overlapping controls,
 query in this knowledge base on the following words: 

    overlapping and controls and Visual and Basic

 Steps to Reproduce Problem
 -------------------------- 

 1. From the File menu, choose New Project (ALT, F, P).
 2. Add a picture control (Picture1) to the default form (Form1). 
 3. Add a command button (Command1) to Form1. 
 4. Add a vertical scroll bar (VScroll1) to Form1. 
 5. Using the mouse, double-click on Form1 to bring up the code 
    window. 
 6. Within the Resize event procedure of Form1, add the following code: 
      Sub Form_Resize () 
          Picture1.Move 0, 0, ScaleWidth - VScroll1.Width, _ 
                        ScaleHeight - Command1.Height 
          VScroll1.Move ScaleWidth - VScroll1.Width, 0, _ 
                        VScroll1.Width, ScaleHeight - Command1.Height 
          Command1.Move 0, ScaleHeight - Command1.Height, _ 
                        ScaleWidth, Command1.Height 
      End Sub 
 Note: The underscores (_) in the above code example indicate that 
 the line should be concatenated with the next line in the Visual Basic 
 environment (VB.EXE). 
 7. Run the program. 
 8. Using the mouse, resize the form by extending the bottom or right 
    sides. When the bottom edge of the form is extended, the command 
    button (Command1) will not refresh. When the right edge of Form1 
    is extended, the scroll bar will not refresh. The refresh problems 
    are caused because Picture1 is expanded and temporarily overlaps 
    the control. When the control (VScroll1 or Command1) is moved out 
    of the way, it is not refreshed. 
 To work around this behavior, use the Refresh method for Picture1, 
 VScroll1, and Command1 after the controls have be moved. Add the 
 following statements to Sub Form_Resize (after the Command1.Move 
 statement) above to overcome the behavior: 
    Picture1.Refresh 
    VScroll1.Refresh 
    Command1.Refresh 
 Additional reference words: 1.00 

COPYRIGHT Microsoft Corporation, 1991.