Allow customizing scrollbar color and size for full screen mode

[PetarKirov]

Also, on Windows 10, the window border and scrollbar do not take the AppsUseLightTheme DWORD in HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Themes\Personalize into account, so there is no way to visually match the selected theme in the Options/Looks menu.

For me personally, customizing the scrollbar size is not as high priority as customizing its color, though it would be nice if I could make it look like the one on recent versions of Gnome Terminal:

[mintty]

Unfortunately, MS makes it really cumbersome to get such adaption accomplished. Rather than adjust GUI elements automatically to themes, they require programmers to do complex and unintuitive extra work to enable this kind of behaviour. If you provide some guidance how to achieve it at the Windows API, I'll gladly tweak that.

[mintty]

As documented, the windows message WM_CTLCOLORSCROLLBAR should facilitate setting the scrollbar look by returning a "brush". However, mintty does not receive this message...

[BrianInglis]

FYI:
https://msdn.microsoft.com/en-us/library/windows/desktop/bb787573(v=vs.85).aspx
"The WM_CTLCOLORSCROLLBAR message is never sent between threads; it is only sent within the same thread."
and:
"The WM_CTLCOLORSCROLLBAR message is used only by child scroll bar controls.
Scrollbars attached to a window (WS_SCROLL and WS_VSCROLL) do not generate this message.
To customize the appearance of scrollbars attached to a window, use the flat scroll bar functions."

[mintty]

Thanks, Brian, for reading that description more carefully than I did...
This makes me wonder once more why this Windows API is so incredibly insane.
So the task for someone feeling enthusiastic about this feature would be to find out what a "flat scroll bar" is and how to incorporate it in mintty.

[BrianInglis]

From other info I read, ignore the "flat" as those functions are no longer supported, just setup or modify the properties of your window scroll bars using the regular scroll bar functions.

[mintty]

Which scroll bar functions? Right now, mintty only enables a scroll bar with the WS_VSCROLL "style" and listens to some scrollbar events ("messages").

[mintty]

How is this actually related to full-screen mode (issue title)?

[BrianInglis]

Sorry I thought I had already replied to comment .-2 - see Windows Scroll Bar Functions at https://msdn.microsoft.com/en-us/library/windows/desktop/ff486021(v=vs.85).aspx
On the previous comment, from the original issue and screenshot, full screen mode seems irrelevant, unless applying the scrollbar properties to be consistent with the Windows theme has a more significant visual impact in full screen mode?

[mintty]

OK, but those functions do not offer any way to customize the scrollbar appearance.

[BrianInglis]

So mintty would have to add support for either desktop themes, or DIY user-draw control?

[mintty]

"Would" - if you consider it worth the effort.
It is insane that desktop applications need to actively "add support for desktop themes" in the first place, rather than getting them automatically applied to any window decoration that is set up with the basic Windows API functions. I am not motivated to investigate for a workaround for such serious Windows design errors.