Geoff Chappell - Software Analyst
As noted in the overview of List-View backgrounds, a List-View control that has no background colour delegates its simple background to its parent window. For the parent, this is an opportunity to customise the control’s background. For instance, someone might like to make a chequerboard of the rows and columns in Details view, and even then to let that be watermarked. There is no stopping some people with decorations, and it is arguably the business of a software platform to help, or at least not to get in the way. It should not surprise if programmers have taken to implementingin the parent window with the idea of doing quite elaborate drawing in the control. They may need care, however.
The parent window, like any window, may receive amessage for all sorts of reasons, most notably to draw its own background. The message comes with no formal indication of why it has been sent, and the ideal is presumably that the parent needn’t care. It can redraw all of its background, using the device context (DC) that comes as a message parameter, and leave it to the DC to determine how much of this redrawing gets to show. The parent window’s natural expectation therefore is to use its own client-area coordinates when working with a DC from a message (or , etc). Indeed, this expectation is so natural that Microsoft’s documentation seems nowhere to mention that a DC sent with such messages might have window coordinates that are not already aligned with the target window’s client area, and no code samples in the SDK check for the possibility.
When ahas come from a List-View control that has no background colour and wants its background drawn by its parent, the DC that is sent with the message has been prepared for the control. Its device-coordinate origin is, naturally enough, at the top left of the control’s client area. The problem for the parent is that until version 6.0, this DC’s window coordinates are also aligned to the control, not to the parent.
This is surely a coding error, and must be in the running for some sort of prize for having been left so long. Especially embarrassing is that COMCTL32 versions from as long ago as the original Windows 95 have a routine (named CCForwardEraseBackground according to Microsoft’s published symbol files) that deals with exactly this problem that forwarding a message to a parent window requires a temporary adjustment of the DC’s window coordinates to fit the parent’s expectation. The code in this routine is unchanged all the way to version 6.10 for Windows Vista. Copies, without the CC prefix to their names, have found their way elsewhere, including into Internet Explorer. What seems to have happened is that when this routine was coded for COMCTL32 version 4.0, nobody noticed that List-View controls were already forwarding (since at least version 3.50) and were just as much affected by what this routine addresses. I can’t help observing that it would have been more easily noticed, even by Microsoft’s own programmers, were it documented. Even today, I can’t find formal documentation that a List-View control with no background colour sends to its parent window.
Starting with version 6.0, when a LVS_EX_TRANSPARENTBKGND extended style (in versions 6.10 and higher).message is sent from a List-View control to its parent window, the DC that is sent with the message has window coordinates aligned to the parent’s client area. This is true also of the message that is sent if the List-View has the
If the change for version 6.0 is thought to be a bug fix, then why is it not applied to version 5.82? After all, this version is distributed simultaneously with versions 6.0 and 6.10, and although the version number seems fixed at 5.82, the code is obviously not frozen. Are successive builds of version 5.82 not to benefit from Microsoft’s continuing discovery of bugs? Anyway, versions 5.82 and 6.0 were first released simultaneously, for Windows XP. What can Microsoft be playing at with fixing bugs in one but not the other? Perhaps the question is best dismissed by inferring that the change must not be regarded by Microsoft as fixing a defect. Yet the change is made, and can affect the programmers to whom Microsoft promotes Windows as a platform for software development, so ought it not be documented as an intended difference?