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shackle: Enforce rules for popup windows


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22424 shackle.el


shackle gives you the means to put an end to popped up buffers not behaving they way you'd like them to. By setting up simple rules you can for instance make Emacs always select help buffers for you or make everything reuse your currently selected window.


Install from MELPA (stable) via M-x package-install RET shackle or download shackle.el, place it into a suitable location such as ~/.emacs.d/vendor/ and add the following to your init file:

(add-to-list 'load-path (expand-file-name "~/.emacs.d/vendor/"))


First you need to customize shackle-rules, this can be done via M-x customize-group RET shackle or in your init file.

As the name of the variable suggests, it's a list of rules. Each rule consists of a condition and a set of key-value combinations that tell what to do with the buffer in question.

The condition can be either a symbol, a string or a list of either. A symbol is interpreted as the major mode of the buffer to match, a string as the name of the buffer (which can be turned into regexp matching by using the :regexp key with a value of t in the key-value part) and a list groups either symbols or strings (as described earlier) while requiring at least one element to match. It's possible to supply a custom predicate by using (:custom function) as condition. The predicate is called with the buffer to be displayed and is interpreted as a match given a non-nil return value.

The following key-value pairs are available:

Select the popped up window. The shackle-select-reused-windows option makes this the default for windows already displaying the buffer.

Special buffers usually have q bound to quit-window which commonly buries the buffer and deletes the window. This option inhibits the latter which is especially useful in combination with :same (as q deleting the reused window is weird behavior for more than one visible window), but can also be used with other keys like :other as well.

Override with a custom action. The specified function is called with BUFFER-OR-NAME, ALIST and PLIST as argument and must return the window to be displayed or nil to inhibit its display. It's possible to reuse existing actions as defined in the sources, but dispatch based on more specific conditions such as the currently opened windows or selected buffer.

Skip handling the display of the buffer in question. Keep in mind that while this avoids switching buffers, popping up windows and displaying frames, it does not inhibit what may have preceded this command, such as the creation or update of the buffer to be displayed.

Reuse the window other-window would select if there's more than one window open, otherwise pop up a new window. When used in combination with the :frame key, do the equivalent with other-frame or a new frame.

Display buffer in the current window.

Pop up a new window instead of displaying the buffer in the current one.

Align a new window at the respective side of the current frame or with the default alignment (customizable with shackle-default-alignment) by splitting the root window. If a function is specified, it is called with zero arguments and must return any of the above alignments.

Aligned window use a default ratio of 0.5 to split up the original window in half (customizable with shackle-default-size), the ratio can be changed on a per-case basis by providing a different floating point value between 0 and 1. A value of 0.33 for example would make it occupy a third of the original window's size. Alternatively you can use an integer value of 1 or greater to display a window of the specified width or height instead. If a function is specified, it is called with zero arguments and must return a number of the above two types.

Pop buffer to a frame instead of a window.

A default rule can be set up by customizing shackle-default-rule. Its format follows the plist as used by shackle-rules and the default rule is used in case none of the rules in shackle-rules yield a match. To have an exception to the default rule, you can use the condition of your choice and either don't list the key-value pair at all, use a different value (like nil for the keys taking boolean values) or use a placeholder key with any value (like :noselect instead of :select). This is merely done to clearly indicate the purpose of the respective rule, not following this recommendation is another fine option.

Once you're done customizing shackle-rules, use M-x shackle-mode to enable shackle interactively. To enable it automatically on startup, add (shackle-mode) to your init file.


The above section expressed as EBNF:

RULES = "(" , { RULE } , ")" .
RULE = "(" , CONDITION , PLIST , ")" .
DEFAULT_RULE = "(" , PLIST , ")" .

FUNCTION_CONDITION = "(:custom" , FUNCTION , ")" .
T_OR_NIL = "t" | "nil" .

PLIST = "(" , [ ":regexp" , T_OR_NIL ] , ACTIONS , ")" .

IGNORE_ACTION = ":ignore" , T_OR_NIL .
OTHER_ACTION = ":other" , T_OR_NIL , [":frame" , T_OR_NIL] .
POPUP_ACTION = ":popup" , T_OR_NIL .
SAME_ACTION = ":same" , T_OR_NIL .
ALIGN_ACTION = ":align" , ALIGN_VALUE , [":size" , SIZE_VALUE] .
ALIGN_VALUE = T_OR_NIL | "above" | "below" | "left" | "right" | FUNCTION .
FRAME_ACTION = ":frame" , T_OR_NIL .

SELECT_ACTION = ":select" , T_OR_NIL .
INHIBIT_WINDOW_QUIT_ACTION = ":inhibit-window-quit" , T_OR_NIL .


In case your rules don't have any effect on a package, you can enable tracing of calls to display-buffer and other functions using it with M-x shackle-trace-functions, perform the action displaying the buffer and check the *shackle trace* buffer for the displayed buffer. If nothing shows up, the package isn't using display-buffer at all, there isn't much you can do in that case other than asking its author to reconsider using it. If it does, one of the following might be the case:


The following example configuration enables the rather radical behavior of always reusing the current window in order to avoid unwanted window splitting:

(setq shackle-default-rule '(:same t))

This one on the other hand provides a less intrusive user experience to select all windows by default unless they are spawned by compilation-mode and demonstrates how to use exceptions:

(setq shackle-rules '((compilation-mode :noselect t))
      shackle-default-rule '(:select t))

My final example tames Helm windows by aligning them at the bottom with a ratio of 40%:

(setq helm-display-function 'pop-to-buffer) ; make helm play nice
(setq shackle-rules '(("\\`\\*helm.*?\\*\\'" :regexp t :align t :size 0.4)))

Breaking Changes


shackle adds an extra entry to display-buffer-alist, a customizable variable in Emacs that specifies what to do with buffers displayed with the display-buffer function. It's used by quite a lot of Emacs packages, including very essential ones like the built-in help and compilation package.

This means other Emacs packages that neither use the display-buffer function directly nor indirectly won't be influenced by shackle. If you should ever come across a package that ought to use it, but doesn't conform, chances are you'll have to speak with upstream instead of me to have it fixed. Another thing to be aware of is that if you've set up a fallback rule, it may take over the Emacs defaults which can play less well with packages (such as Magit or Helm). Once you find out what's causing the problem, you can add an exception rule to fix it.


This package assumes that every case of altering the buffer display rules can be caught by checking for the buffer name or major mode of the respective buffer. While this is true in most cases, there are obviously exceptions to this rule. For example find-function-at-point ends up displaying a file buffer containing the function definition in another window, but you can't infer this from that buffer alone. The simple workaround is just replacing find-function-at-point with something directly using your preferred flavor of display-buffer. If you're hell-bent on making it work with shackle though, you could check whether using custom conditions/actions works for you. In case they aren't enough, advise the function displaying the buffer to alter it so that it can be detected by them.


This package is heavily inspired by popwin and was hacked together after discovering it being hard to debug, creating overly many timers and exposing rather baffling bugs. shackle being intentionally simpler and easier to understand is considered a debugging-friendly feature, not a bug. However if you prefer less rough edges, a sensible default configuration and having more options for customizing, give popwin a try.