A taskbar is an element of a graphical user interface which has various purposes. It typically shows which programs are currently running.
The specific design and layout of the taskbar varies between individual operating systems, but generally assumes the form of a strip located along one edge of the screen. On this strip are various icons which correspond to the windows open within a program. Clicking these icons allow the user to easily switch between programs or windows, with the currently active program or window usually appearing differently from the rest. In more recent versions of operating systems, users can also "pin" programs or files so that they can be accessed quickly, often with a single click. Due to its prominence on the screen, the taskbar usually also has a notification area, which uses interactive icons to display real-time information about the state of the computer system and some of the programs active on it.
With the rapid development of operating systems and graphical user interfaces in general, more OS-specific elements have become integrated into and become key elements of the taskbar.
Windows 1.0, released in 1985, features a horizontal bar located at the bottom of the screen where running programs reside when minimized (referred to as "iconization" at the time), represented by icons. A window can be minimized by double-clicking its title bar, dragging it onto an empty spot on the bar, or by issuing a command from one of its menus. A minimized window is restored by double-clicking its icon or dragging the icon out of the bar.
The bar features multiple slots for icons and expands vertically to provide the user with more rows as more slots are needed. Its color is the same as that of the screen background, which can be customized. Minimized windows can be freely placed in any of the empty slots. Program windows cannot overlap the bar unless maximized.
The Start button did not make an appearance in these early implementations of the taskbar, and would be introduced at a much later date with the release of Windows 95.
Another early implementation can be seen in the Arthur operating system from Acorn Computers. It is called the icon bar and remains an essential part of Arthur's succeeding RISC OS operating system. The icon bar holds icons which represent mounted disc drives and RAM discs, running applications and system utilities. These icons have their own context-sensitive menus and support drag and drop behaviour.
AmigaOS featured various third party implementations of the taskbar concept, and this inheritance is present also in its successors. For example, AmiDock, born as third-party utility, has then been integrated into AmigaOS 3.9 and AmigaOS 4.0. The AROS operating system has its version of Amistart that is provided with the OS and free to be installed by users, while MorphOS has been equipped with a dock utility just like in AmigaOS or Mac OS X.
The default settings for the taskbar in Microsoft Windows place it at the bottom of the screen and includes from left to right the Start menu button, Quick Launch bar, taskbar buttons, and notification area. The Quick Launch toolbar was added with the Windows Desktop Update and is not enabled by default in Windows XP. Windows 7 removed the Quick Launch feature in favor of pinning applications to the taskbar itself. On Windows 8 and Windows Server 2012, a hotspot located in the bottom-left corner of the screen replaced the Start button, although this change was reverted in Windows 8.1 and Windows Server 2012 R2.
The taskbar was originally developed as a feature of Windows 95, but it was based on a similar user interface feature called the tray that was developed as part of Microsoft's Cairo project.
With the release of Windows XP, Microsoft changed the behavior of the taskbar to take advantage of Fitts's law by removing a border of pixels surrounding the Start button which did not activate the menu, allowing the menu to be activated by clicking directly in the corner of the screen.
The Windows taskbar can be modified by users in several ways. The position of the taskbar can be changed to appear on any edge of the primary display (except in Windows 11, where the taskbar is permanently fixed at the bottom of the screen and cannot be moved to the top, left, or right side). Up to and including Windows Server 2008, the taskbar is constrained to single display, although third-party utilities such as UltraMon allow it to span multiple displays. When the taskbar is displayed vertically on versions of Windows prior to Windows Vista, the Start menu button will only display the text "Start" or translated equivalent if the taskbar is wide enough to show the full text. However, the edge of the taskbar (in any position) can be dragged to control its height (width for a vertical taskbar); this is especially useful for a vertical taskbar to show window titles next to the window icons.
Users can resize the height (or width when displayed vertically) of the taskbar up to half of the display area. To avoid inadvertent resizing or repositioning of the taskbar, Windows XP and later lock the taskbar by default. When unlocked, "grips" are displayed next to the movable elements which allow grabbing with the mouse to move and size. These grips slightly decrease amount of available space in the taskbar.
The taskbar as a whole can be hidden until the mouse pointer is moved to the display edge, or has keyboard focus. The Windows 7+ taskbar does not allow pinning any arbitrary folder to the taskbar, it gets pinned instead to the jumplist of a pinned Explorer shortcut, however third party utilities such as Winaero's Taskbar Pinner can be used to pin any type of shortcut to the Taskbar.
Other toolbars, known as "Deskbands", may be added to the taskbar. This feature, along with many other taskbar features is currently absent in Windows 11. Windows includes the following deskbands but does not display them by default (except the Quick Launch toolbar in certain versions and configurations).
In addition to deskbands, Windows supports "Application Desktop Toolbars" (also called "appbands") that supports creating additional toolbars that can dock to any side of the screen, and cannot be overlaid by other applications.
Users can add additional toolbars that display the contents of folders. The display for toolbars that represent folder items (such as Links, Desktop and Quick Launch) can be changed to show large icons and the text for each item. Prior to Windows Vista, the Desktop Toolbars could be dragged off the taskbar and float independently, or docked to a display edge. Windows Vista greatly limited, but did not eliminate the ability to have desktop toolbar not attached to the taskbar. Windows 7 has deprecated the use of Floating Deskbands altogether; they only appear pinned into the Taskbar.
Classic Mac OS did not display a taskbar onscreen by default. Application switching prior to Mac OS 8.5 was done by clicking on an application's window or via a pull-down menu at the right end of the menu bar. Prior to version 8.5 the menu's title was the icon of the foreground application. Version 8.5 introduced the ability to optionally also display the application name and to "tear off" the menu by dragging the title with the mouse. The torn off menu was displayed as a palette. The palette window could be configured using AppleScript to appear much like a taskbar, with no title bar and fixed to one edge of the screen. No control panel was provided by Apple to access this functionality, but third-party developers quickly wrote applications that allowed users unfamiliar with AppleScript to customize their application palettes. Third party taskbars such as DragThing were a popular category of shareware on these systems.
The Dock, as featured in macOS and its predecessor NeXTSTEP, is also a kind of taskbar. The macOS Dock is application-oriented instead of window-oriented. Each running application is represented by one icon in the Dock regardless of how many windows it has on screen. A textual menu can be opened by right-clicking on the dock icon that gives access to an application's windows. Mac OS X 10.2 added the ability for an application to add items of its own to this menu. Minimized windows also appear in the dock, in the rightmost section, represented by a real-time graphical thumbnail of the window's contents. The trash can is also represented in the Dock, as a universal metaphor for deletion. For example, dragging selected text to the trash should remove the text from the document and create a clipping file in the trash.
The right side of macOS's Menu bar also typically contains several notification widgets and quick access functions, called Menu extras.
In KDE Plasma 5, taskbar uses Widgets, called "Plasmoids", as elements in taskbar. In the update 5.20 (November 2020) they updated the taskbar to look more like Windows 10 by only displaying icons by default and grouping application windows together. 
GNOME 2 used its own type of taskbar, known as panels (the program responsible for them is therefore called gnome-panel). By default, GNOME 2 usually contains two full-width panels at the top and bottom of the screen. The top panel usually contains navigation menus labeled Applications, Places, and System in that order. These menus hold links to common applications, areas of the file system, and system preferences and administration utilities, respectively.
The top panel usually contains a clock and notification area, while the bottom panel contains buttons for navigating between virtual desktops, the window list proper, and a button which minimizes all windows (similarly to Windows' Show desktop button). The contents of panels are handled by widgets called panel applets, which can consist of application shortcuts, search tools, or other tools. The contents of the panels can be moved, removed, or configured in other ways.
In GNOME 3, panels are replaced by GNOME Shell, which consists of a bar across the top of the screen with an Activities button on the left, a clock in the centre, and a notification area on the right. GNOME Shell does not contain a traditional taskbar; users can manage windows, virtual desktops, and launch applications from either a "Dash" on the side of the screen, or by searching from Activities Overview, which is displayed by clicking on the Activities button. GNOME 3.8 introduces Classic Mode, which re-implements certain aspects of GNOME 2's desktop as an alternate desktop environment that can be selected at the login screen. 
These desktop environments provide their own implementation of a taskbar:
Standalone window managers that provide an integrated taskbar include:
Programs that offer standalone taskbars for desktop environments or window managers without one include Avant Window Navigator, pypanel, fbpanel, perlpanel, tint2, and others.
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