CHl’86 Proceedinas Am-it 1986
A Comparison of Tiled and Overlapping Windows
Sara A. Bly Jarrett K. Rosenberg
Xerox Office Systems Division
2100 Geng Road
Palo Alto. CA 94303
Abstract
It is widely believed that overlapping windows are
preferable to tiled (non-overlappingi ones, but there is
very little research to Support that belief. An analysts of
the basic characteristics of windowing reg$nes predicts
that there are. in fact. situations where overlapping
windows are inferior to tiled. An experiment to test this
prediction verified that there are rndeed tasks and users
for which tiled windows yield faster performance. This
result suggests a need for closer study of the principles
underlying. windowing regimes, so that designers havea
better understanding of the tradeoffs involved in using
them.
1. Introduction
The current trend toward the use of overlapping
windows in multi-window systems is based on the
assumption that overlapping windows are clearly more
beneficial to users than tiled ones. However, there is
very little analysis or experimentation on window
systems (though see Card et al., 1984; Cohen et al.,
1985; Bury et al., 1985). This lack of information
makes it difficult to understand the relative value of
different window mangagement systems. In this paper
we present a simple analysis of windowing regimes
and the results of an experiment to test predictions
derived from the analysis.
2. An analyis of windowing regimes.
The fundamental characteristics of a multi-window
system are 1) its mechanisms for window location and
size, 2) the effects of manipulating one window on
other windows, and 3) the degree of user or system
control of these parameters. We define a tiled window
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system as one in which any open window is always
fully visible; windows are not aliowed to overlap. The
system attempts to manage the window locations,
sizes, and side-effects to maximize the use of the
screen space while keeping window contents visible.
Such a system typically determines the location and
size of each window. When the location and/or size of
a window changes, other windows are relocated and
resized as needed, but never obscured. Since
window size must decrease with an increase in the
number of windows, a user often does not see much
of the contents of a newly opened window. The
decreasing size also means that a tiled system limits
the number of windows that may be open
simultaneously. Examples of tiled window systems
include the Star system (Smith et al., 1982a,b), the
Cedar system (Teiteiman, 1984), the Andrew system
(ITC, 1984), and Microsoft windows (Lemmons,
1983b).
We define an overlapping window system as one in
which the user manages a window’s location and size
in any way desired. Thus, the user controls the use of
the screen space and the visibility of window contents.
When the location and/or size of a window changes,
other windows may be obscured, but their locations
and sizes do not change. Because windows may
overlap, a user can always choose to see a full
screen’s worth of contents of a newly opened window.
Examples of overlapping window systems include
various programming environments, e.g., Smalltalk
(Tesler, 1981) and Interlisp- (Sheil, 1983), as well
as the Vision system (Lemmons, 1983a) and WHIM
(Goodfellow, 1985).
User requirements for a multi-window system can
be characterized as
l the ability of windows to conform to their
contents so as to ‘maximize the the visibility of
those contents, and
0 the ability of the system to relieve the user of
having to manage the size and location of the
windows.
Overlapping window systems maximize the first
user requirement. In an overlapping system, the user
101

CH 1’86 Proceedings April 1986
has control over moving, resizing, and overlapping
windows anywhere on the screen. Such systems
typically do very little to aid the user in window
management. Tiled window systems maximize the
second user requirement. Because most tiled
systems attempt to satisfy the conformance
requirement by using all of the available screen space,
windows often change size and focation when other
windows are opened or closed. Tiled systems
typically do very little to conform to window contents
that are not of a default shape, and as the number of
open windows increases, the size of each window
decreases.1
3. Some predictions.
From the analysis above, we can predict situations
in which one or the other window system is preferable.
A tiled system aids a user when a) the contents of the
windows basically conform to the defined arrangement
of window size and placement, or b) the user does not
wish to be distracted by managing the windows. An
overlapping system aids a user when a) the contents
of the windows do not conform to any pre-
determined window arrangement, or b) the user
wishes to control the window environment completely.
A task in which we expect a tiled window system to
aid user performance would require that the contents
of the windows have maximum visibility within the
default window location and size (see Figure 1). A
user would have no need for additional window
management and could proceed directly with the task.
Performing the same task in an overlapping system
would require that the user manipulate the windows
into a state similar to that automatically provided in a
tiled system.
I I
\ /
Figure 1. A task suitable for tiled windows.
Conversely, a task in which we would expect an
overlapping system to aid user performance would
involve windows whose contents did not conform to a
regular pattern (see Figure 2). In order to obtain
1 Note that systems which maximize neither requirement are
unusable, and those which maximize both must be prescient to a
degree unattainable by current techniques.
maximum visibility of the contents, the user would
have to arrange the windows in some irregular
manner, utilizing the ability to manage and overlap the
windows.
Figure 2. A task suitable for overlapping.
However, even with a task which required window
manipulation in order to be performed efficiently,
overlapping windows might not aid users unskilled in
the use of the window management mechanisms.
Such inexpert users might spend more effort in trying
to master the functionality of overlapping windows than
in scrolling and opening closing the windows of a tiled
sytem.
4. An experiment.
In order to test the prediction that different window
systems may be preferable for different tasks we
conducted an experiment using the Xerox Viewpoint
Office System (a direct descendant of the Star
system). The Viewpoint system was useful because
the user may specify whether window management is
tiled or overlapping without affecting other aspects of
the system.
The Viewpoint tiled window system allows at most
six open windows, three vertically on each half of the
screen. When a user opens a window, it will open In
the next available space. As a window opens, it
divides the vertical space equally with the other
windows on the same side of the screen (see Figure
3). The user may control the height of a window and
whether the window is on the left or right half of the
screen. In general, when a window is closed,
remaining open windows attempt to fill the vacated
space. If the user has manipulated windows by
changing their lengths or positions in the vertical
column, opening additional windows may cause the
system to override those changes. As subsequent
opportunities occur for re-satisfying the user-
specified constraints, the window management system
attempts to do so.
The Viewpoint overlapping window system provides
the user with controls for moving a window, resizing a
window (both width and height), and placing a window
102