Buffer-local `scad-extra-args` not respected in preview (wrong buffer context)

[rubenphilipp]

When setting scad-extra-args buffer-locally in a scad-mode buffer (e.g. via .dir-locals.el, to pass project-specific -D variables), the setting has no effect on scad-preview.

Root cause: scad--preview-render executes with current-buffer bound to the *scad preview: ...* buffer (scad-preview-mode), not the originating scad-mode source buffer. Buffer-local variables set for scad-mode are therefore invisible when the command is assembled, and the global default (nil) is used instead.

Motivating use case: import() of external mesh files (STL etc.) resolves relative paths against the actual script file location — but scad--preview-render always writes the buffer to a fresh temp file elsewhere (make-temp-file), so relative imports always fail in preview even though they work fine when opening the file directly in the OpenSCAD app. Passing a project-relative -D project_dir="/abs/path" from the source buffer would be a reasonable workaround, but currently can't reach the preview process due to the buffer-context mismatch above.

Working local workaround (via advice, since scad--preview-render is private):

(advice-add 'scad--preview-render :around
            (lambda (orig-fn &rest args)
              (let* ((src-file (and (buffer-live-p scad--preview-buffer)
                                     (buffer-local-value 'buffer-file-name scad--preview-buffer)))
                     (scad-extra-args
                      (if src-file
                          (append (list "-D" (format "project_dir=\"%s\""
                                                      (directory-file-name (file-name-directory src-file))))
                                  scad-extra-args)
                        scad-extra-args)))
                (apply orig-fn args))))

Suggested fix: When assembling scad-extra-args in scad--preview-render, additionally read the buffer-local value from scad--preview-buffer (the source buffer), not just from the current (preview) buffer — e.g. via buffer-local-value.

Environment

• scad-mode version: 20260519.1039 (revision 8798dca)
• Emacs: GNU Emacs 30.2 (build 1, aarch64-apple-darwin25.4.0), macOS 26.5.1