[PATCH 1/2] comctl32/listbox: Don't update scrollbar during window creation

[Fabian Maurer]

On Freitag, 14. September 2018 14:14:07 CEST Nikolay Sivov wrote:
> What is the issue exactly?

As the tests show, a listbox with WS_VSCROLL and height 0 has the WS_VSCROLL style set 
after window creation.

However, when LISTBOX_UpdateScroll is called, this hides the scrollbar (removing the 
WS_VSCROLL style) - since it's not needed when the listbox is empty.

Now, CreateWindowEx sends WM_SIZE with SIZE_RESTORED, this triggers 
LISTBOX_UpdateSize which then calls LISTBOX_UpdateScroll.
CreateWindowEx also sends WM_SETFONT which will trigger a resize - leading to the same 
issue.

But as the tests also show, this is only relevant during CreateWindowEx. When with listbox 
empty and height 0, causing a WM_SIZE due to SetWindowPos causes the scrollbar to be 
hidden.

I didn't find a better solution yet, since it indeed seems to only be problematic during 
initial window creation.

FWIW, the issue is themed delphi programs, their comboboxes are *very* sensitive about 
listbox height/WS_VSCROLL style when WM_NCCALCSIZE is sent.

Regards,
Fabian Maurer


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