This details the name of this window style and the options available in .NET; see @Synetech's excellent answer for the original purpose of this window style, the reasons it may be used for such a dialog box and the implementation in unmanaged Windows programs such as Notepad++.
The window has square corners and looks like that because one of the designers/programmers of Notepad++ decided to use a specific window style. Synetech details the advantages of that window style.
In .NET/Visual Studio, this is set in the
FormBorderStyle property of the
System.Windows.Forms.Form class. Specifically, it is the
FixedToolWindow. The possible values are in the
ToolWindows in .NET with all values default work a little differently from a plain
WS_EX_TOOLWINDOW in the Windows API, though they look the same. With testing, I have determined that they do appear on the taskbar and they do appear on the Alt+Tab task switcher unless the form property
ShowInTaskbar is set to
ShowInTaskbar affects visibility in both the taskbar and Alt+Tab for
ToolWindows, but only taskbar for others.
Meanwhile, it is also possible to remove the icon/minimise/restore/maximise buttons from the more common window styles, though it does not seem to be possible to add them to
ToolWindows. It is also possible to hide normal windows from the taskbar.
The only functional advantage
WS_EX_TOOLWINDOW offers, as far as I can tell, is that the window does not appear in Alt+Tab.
I have included some screenshots comparing the .NET window styles. These are not directly used by Notepad++, nor other unmanaged programs, but are named by Microsoft, so...
Possible values of
The first image is a screenshot of the running program with Aero enabled, the second is from the Visual Studio designer view (no Aero).
Click the images for the full size versions
Descriptions are taken from the MSDN article on the
A fixed, single-line border.
A fixed, three-dimensional border.
A thick, fixed dialog-style border.
A resizable border.
A tool window border that is not resizable. A tool window does not appear in the taskbar or in the window that appears when the user presses ALT+TAB. Although forms that specify
FixedToolWindow typically are not shown in the taskbar, you must also ensure that the
ShowInTaskbar property is set to
false, since its default value is
A resizable tool window border. A tool window does not appear in the taskbar or in the window that appears when the user presses ALT+TAB.