bug#34022: 26.1; Region highlight has lower priority than flymake overlays

[Johann Klähn]

I just noticed a bug (?) that region becomes de-facto "invisible" due to its 
highlight overlay having a lower priority than the flymake overlays. This only 
happens when both region and flymake diagnostics are indicated using the same 
face property (background in my case). It becomes more apparent for larger 
flymake overlays.

Steps to reproduce, starting from emacs -Q:

• M-: (set-face-background flymake-note "yellow green")
• M-x find-library flymake (or any file where there will be flymake diagnostics)
• M-x flymake-mode
• Jump to a docstring/line/paragraph highlighted using e.g. the `flymake-note' 
face.
• With transient-mark-mode enabled select regions that overlap with the
  flymake overlay. ⇒ The region is not highlighted since the background
 face property given by the flymake overlay takes precedence.

Relevant extract from simple.el.gz, where the secondary priority for
the region highlight overlay is set to 100:

> (defvar redisplay-highlight-region-function
>   (lambda (start end window rol)
>     (if (not (overlayp rol))
>         (let ((nrol (make-overlay start end)))
>           (funcall redisplay-unhighlight-region-function rol)
>           (overlay-put nrol 'window window)
>           (overlay-put nrol 'face 'region)
>           ;; Normal priority so that a large region doesn't hide all the
>           ;; overlays within it, but high secondary priority so that if it
>           ;; ends/starts in the middle of a small overlay, that small overlay
>           ;; won't hide the region's boundaries.
>           (overlay-put nrol 'priority '(nil . 100))
>           nrol)
>       (unless (and (eq (overlay-buffer rol) (current-buffer))
>                    (eq (overlay-start rol) start)
>                    (eq (overlay-end rol) end))
>         (move-overlay rol start end (current-buffer)))
>       rol)))

Flymake uses 100 as the offset to compute the default priority for its overlays:

> (defun flymake--highlight-line (diagnostic)
>   "Highlight buffer with info in DIAGNOSTIC."
>   (when-let* ((ov (make-overlay
>                    (flymake--diag-beg diagnostic)
>                    (flymake--diag-end diagnostic))))
>     ;; First set `category' in the overlay, then copy over every other
>     ;; property.
>     ;;
>     (let ((alist (assoc-default (flymake--diag-type diagnostic)
>                                 flymake-diagnostic-types-alist)))
>       (overlay-put ov 'category (assoc-default 'flymake-category alist))
>       (cl-loop for (k . v) in alist
>                unless (eq k 'category)
>                do (overlay-put ov k v)))
>     ;; Now ensure some essential defaults are set
>     ;;
>     (cl-flet ((default-maybe
>                 (prop value)
>                 (unless (or (plist-member (overlay-properties ov) prop)
>                             (let ((cat (overlay-get ov
>                                                     'flymake-category)))
>                               (and cat
>                                    (plist-member (symbol-plist cat) prop))))
>                   (overlay-put ov prop value))))
>       (default-maybe 'bitmap 'flymake-error-bitmap)
>       (default-maybe 'face 'flymake-error)
>       (default-maybe 'before-string
>         (flymake--fringe-overlay-spec
>          (overlay-get ov 'bitmap)))
>       (default-maybe 'help-echo
>         (lambda (window _ov pos)
>           (with-selected-window window
>             (mapconcat
>              #'flymake--diag-text
>              (flymake-diagnostics pos)
>              "\n"))))
>       (default-maybe 'severity (warning-numeric-level :error))
>       (default-maybe 'priority (+ 100 (overlay-get ov 'severity))))
>     ;; Some properties can't be overridden.
>     ;;
>     (overlay-put ov 'evaporate t)
>     (overlay-put ov 'flymake-diagnostic diagnostic)))

Is it intentional that 100 is used as the offset here? This has the effect that 
most priorities will be above that of the region overlay.

---

In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
 of 2018-07-05 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
Recent messages:

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-modules
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong
 -fno-plt' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

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Major mode: Emacs-Lisp

Minor modes in effect:
  flymake-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
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  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
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  auto-encryption-mode: t
  auto-compression-mode: t
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  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

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