Introduce new value t for compilation-context-lines to eliminate scrolling
In particular, to prevent scrolling in a window lacking a left fringe. Instead, a visible arrow "=>" is inserted before column zero. This fixes bug #36832. * lisp/progmodes/compile.el (compilation-context-lines): Add the new value t. (compilation-set-window): Amend to handle compilation-context-lines being t. (overlay-arrow-overlay): New variable holding an overlay with before-string property "=>". (compilation-set-overlay-arrow): New function which manipulates overlay-arrow-overlay. (compilation-goto-locus, compilation-find-file): In addition to calling compilation-set-window, also call compilation-set-overlay-arrow. * doc/emacs/building.texi (Compilation Mode): Document the new value t which compilation-context-lines can take. * etc/NEWS: Add an entry for this change.
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