29.14 Child Frames

Child frames are objects halfway between windows (see Windows) and “normal” frames. Like windows, they are attached to an owning frame. Unlike windows, they may overlap each other—changing the size or position of one child frame does not change the size or position of any of its sibling child frames.

By design, operations to make or modify child frames are implemented with the help of frame parameters (see Frame Parameters) without any specialized functions or customizable variables. Note that child frames are meaningful on graphical terminals only.

To create a new child frame or to convert a normal frame into a child frame, set that frame's parent-frame parameter (see Frame Interaction Parameters) to that of an already existing frame. The frame specified by that parameter will then be the frame's parent frame as long as the parameter is not changed or reset. Technically, this makes the child frame's window-system window a child window of the parent frame's window-system window.

The parent-frame parameter can be changed at any time. Setting it to another frame reparents the child frame. Setting it to another child frame makes the frame a nested child frame. Setting it to nil restores the frame's status as a top-level frame—a frame whose window-system window is a child of its display's root window.

Since child frames can be arbitrarily nested, a frame can be both a child and a parent frame. Also, the relative roles of child and parent frame may be reversed at any time (though it's usually a good idea to keep the size of a child frame sufficiently smaller than that of its parent). An error will be signaled for the attempt to make a frame an ancestor of itself.

Most window-systems clip a child frame at the native edges (see Frame Geometry) of its parent frame—everything outside these edges is usually invisible. A child frame's left and top parameters specify a position relative to the top-left corner of its parent's native frame. When the parent frame is resized, this position remains conceptually unaltered.

NS builds do not clip child frames at the parent frame's edges, allowing them to be positioned so they do not obscure the parent frame while still being visible themselves.

Usually, moving a parent frame moves along all its child frames and their descendants as well, keeping their relative positions unaltered. Note that the hook move-frame-functions (see Frame Position) is run for a child frame only when the position of the child frame relative to its parent frame changes.

When a parent frame is resized, its child frames conceptually retain their previous sizes and their positions relative to the left upper corner of the parent. This means that a child frame may become (partially) invisible when its parent frame shrinks. The parameter keep-ratio (see Frame Interaction Parameters) can be used to resize and reposition a child frame proportionally whenever its parent frame is resized. This may avoid obscuring parts of a frame when its parent frame is shrunk.

A visible child frame always appears on top of its parent frame thus obscuring parts of it, except on NS builds where it may be positioned beneath the parent. This is comparable to the window-system window of a top-level frame which also always appears on top of its parent window—the desktop's root window. When a parent frame is iconified or made invisible (see Visibility of Frames), its child frames are made invisible. When a parent frame is deiconified or made visible, its child frames are made visible. When a parent frame is about to be deleted (see Deleting Frames), its child frames are recursively deleted before it.

Whether a child frame can have a menu or tool bar is window-system or window manager dependent. Most window-systems explicitly disallow menus bars for child frames. It seems advisable to disable both, menu and tool bars, via the frame's initial parameters settings.

Usually, child frames do not exhibit window manager decorations like a title bar or external borders (see Frame Geometry). When the child frame does not show a menu or tool bar, any other of the frame's borders (see Layout Parameters) can be used instead of the external borders.

In particular, under X (but not when building with GTK+), the frame's outer border can be used. On MS-Windows, specifying a non-zero outer border width will show a one-pixel wide external border. Under all window-systems, the internal border can be used. In either case, it's advisable to disable a child frame's window manager decorations with the undecorated frame parameter (see Management Parameters).

To resize or move an undecorated child frame with the mouse, special frame parameters (see Mouse Dragging Parameters) have to be used. The internal border of a child frame, if present, can be used to resize the frame with the mouse, provided that frame has a non-nil drag-internal-border parameter. If set, the snap-width parameter indicates the number of pixels where the frame snaps at the respective edge or corner of its parent frame.

There are two ways to drag an entire child frame with the mouse: The drag-with-mode-line parameter, if non-nil, allows to drag a frame without minibuffer window (see Minibuffer Windows) via the mode line area of its bottommost window. The drag-with-header-line parameter, if non-nil, allows to drag the frame via the header line area of its topmost window.

In order to give a child frame a draggable header or mode line, the window parameters mode-line-format and header-line-format are handy (see Window Parameters). These allow to remove an unwanted mode line (when drag-with-header-line is chosen) and to remove mouse-sensitive areas which might interfere with frame dragging.

To avoid that dragging moves a frame completely out of its parent's native frame, something which might happen when the mouse cursor overshoots and makes the frame difficult to retrieve once the mouse button has been released, it is advisable to set the frame's top-visible or bottom-visible parameter correspondingly.

The top-visible parameter specifies the number of pixels at the top of the frame that always remain visible within the parent's native frame during dragging and should be set when specifying a non-nil drag-with-header-line parameter. The bottom-visible parameter specifies the number of pixels at the bottom of the frame that always remain visible within the parent's native frame during dragging and should be preferred when specifying a non-nil drag-with-mode-line parameter.

When a child frame is used for displaying a buffer via display-buffer-in-child-frame (see Buffer Display Action Functions), the frame's auto-hide-function parameter (see Frame Interaction Parameters) can be set to a function, in order to appropriately deal with the frame when the window displaying the buffer shall be quit.

When a child frame is used during minibuffer interaction, for example, to display completions in a separate window, the minibuffer-exit parameter (see Frame Interaction Parameters) is useful in order to deal with the frame when the minibuffer is exited.

The behavior of child frames deviates from that of top-level frames in a number of other ways as well. Here we sketch a few of them:

• The semantics of maximizing and iconifying child frames is highly window-system dependent. As a rule, applications should never invoke these operations on child frames. By default, invoking iconify-frame on a child frame will try to iconify the top-level frame corresponding to that child frame instead. To obtain a different behavior, users may customize the option iconify-child-frame described below.
• Raising, lowering and restacking child frames (see Raising and Lowering) or changing the z-group (see Position Parameters) of a child frame changes only the stacking order of child frames with the same parent.
• Many window-systems are not able to change the opacity (see Font and Color Parameters) of child frames.
• Transferring focus from a child frame to an ancestor that is not its parent by clicking with the mouse in a visible part of that ancestor's window may fail with some window-systems. You may have to click into the direct parent's window-system window first.
• Window managers might not bother to extend their focus follows mouse policy to child frames. Customizing mouse-autoselect-window can help in this regard (see Mouse Window Auto-selection).
• Dropping (see Drag and Drop) on child frames is not guaranteed to work on all window-systems. Some will drop the object on the parent frame or on some ancestor instead.

The following two functions can be useful when working with child and parent frames:

Note also the function window-largest-empty-rectangle (see Coordinates and Windows) which can be used to inscribe a child frame in the largest empty area of an existing window. This can be useful to avoid that a child frame obscures any text shown in that window.

Customizing the following option can be useful to tweak the behavior of iconify-frame for child frames.