Mu4e: start using thread folding package

It does what it says on the tin, and mu4e doesn't (currently) do this.

modules/email/mu4e/config.el

@@ -214,6 +214,35 @@
   (advice-add 'mu4e~start :around #'+mu4e-lock-start)
   (advice-add 'mu4e-quit :after #'+mu4e-lock-file-delete-maybe))
 
+(use-package! mu4e-thread-folding
+  :after mu4e
+  :config
+  (setq mu4e-thread-folding-root-folded-prefix-string (propertize "▶ " 'face 'shadow)
+        mu4e-thread-folding-root-unfolded-prefix-string (propertize "▼ " 'face 'shadow))
+  (custom-set-faces!
+    '(mu4e-thread-folding-root-unfolded-face :weight bold :slant italic :inherit hl-line :extend t)
+    '(mu4e-thread-folding-child-face :inherit hl-line :extend t))
+  (map! :map mu4e-headers-mode-map
+        :ne "<tab>" #'mu4e-headers-toggle-at-point
+        :ne "<left>" #'mu4e-headers-fold-at-point
+        :ne "<S-left>" #'mu4e-headers-fold-all
+        :ne "<right>" #'mu4e-headers-unfold-at-point
+        :ne "<S-right>" #'mu4e-headers-unfold-all)
+
+  (when (featurep! :editor evil)
+    (defadvice! +mu4e-thread-folding-move-to-column-1-a (&rest _)
+      "Move the point to column 1.
+When using evil, having the cursor at column 0 causes issues,
+so we make sure that it's put a column 1 so everything works nicely."
+      :before #'mu4e-headers-toggle-at-point
+      :before #'mu4e-headers-fold-at-point
+      :before #'mu4e-headers-unfold-at-point
+      :before #'mu4e-headers-view-message
+      :before #'mu4e-compose-reply
+      :before #'mu4e-compose-forward
+      (unless (= (current-column) 1)
+        (move-to-column 1 t)))))
+
 (unless (featurep! +org)
   (after! mu4e
     (defun org-msg-mode (&optional _)

modules/email/mu4e/packages.el

@@ -5,3 +5,6 @@
   (package! org-msg :pin "89e746c0a864031eef940758230bc7263a6f2289"))
 
 (package! mu4e-alert :pin "91f0657c5b245a9de57aa38391221fb5d141d9bd")
+
+(package! mu4e-thread-folding :recipe (:host github :repo "rougier/mu4e-thread-folding")
+  :pin "56bb25a1addba4c6ed79c4e5d1a580d80cc698f2")