Folders one level at a time on a phone, and targets a thumb can hit

Three things the mobile interface got wrong, all of them measurable.

The folder tree spent its width on depth. Four levels down, the 16px
indent steps and the 18px twisty left a folder 85px of a 300px drawer to
print its name in, and the twisty had walked far enough right that
hitting it was luck -- a miss landed on the row, which is a link, so the
wrong tap also cost a navigation. Under 768px the tree is now a
drill-down: one level at a time, no indent, a back row above it, and a
chevron at the right edge that is the same size in the same place on
every row. Tapping the row still opens the folder; only the chevron
changes what the list shows. The tree is untouched above 768px, where a
wide sidebar can afford the indent and where dragging a folder onto
another folder -- still the only way to reparent one -- needs both of
them on screen at once.

Every control in the top bar was under the 44px a fingertip covers: the
icons at 36, the search filter at 30, the row menu at 24, and the
hamburger 6px from the bezel in the corner a thumb is worst at. They
keep the size they draw at and gain a transparent hit area, since
growing the boxes would reflow a bar with no room to give; rows grow for
real, because a 44px target inside a 36px row reaches into its
neighbours. The one exception is the row menu, held to 36px wide: at a
full 44 it overlapped the drill chevron by 4px, so its right edge
silently drilled instead.

Pinch was dead on the message list. `.msg-row` sets `touch-action:
pan-y` to feed the swipe gesture the horizontal movement the browser is
not using -- but naming any value drops every gesture not named, zoom
included. It worked on an open message and died on the list, which reads
as the zoom being broken at random rather than as a rule about rows.
`pan-y pinch-zoom` keeps the swipe and gives the zoom back.

web/src/styles/app.css

@@ -448,6 +448,18 @@ a.menu-item:hover { color: var(--fg); }
 .nav-item.folder-row.depth-4 { --folder-indent: 64px; }
 .nav-item.folder-row .nav-twisty { position: absolute; left: calc(8px + var(--folder-indent)); top: 50%; transform: translateY(-50%); margin: 0; }
 /* Labels sit in the same column: a 20px slot matching the folder icons. */
+/* ---- Drill-down (phones; see mail/MailboxTree.tsx) ---- */
+/* No indent to pay for, so the name gets the width the tree was spending on
+   depth. The twisty's 30px gutter goes too -- nothing is drawn in it here. */
+.nav-item.folder-row.has-drill { padding-left: 12px; }
+.nav-item.folder-row .drill-into { width: 44px; height: 100%; border-radius: 0; margin-right: -12px; color: var(--fg-faint); }
+.nav-item.folder-row .drill-into:hover { background: var(--bg-hover); color: var(--fg); }
+/* The row it belongs to is a link and paints its own active state; a second
+   filled rectangle inside it reads as a separate selected thing. */
+.nav-item.folder-row.active .drill-into { color: inherit; }
+.drill-back { width: 100%; border: 0; background: none; font: inherit; text-align: left; cursor: pointer; color: var(--fg-muted); padding-left: 8px; }
+.drill-back:hover { background: var(--bg-hover); color: var(--fg); }
+.drill-back svg { flex: 0 0 auto; }
 .nav-item.folder-row .nav-label-color { flex: 0 0 20px; width: 20px; height: 20px; border-radius: 0; display: inline-flex; align-items: center; justify-content: center; }
 .nav-item.folder-row .nav-label-color::before { content: ""; width: 11px; height: 11px; border-radius: 3px; background: var(--label-color, var(--accent)); }
 .collapsed .nav-item { justify-content: center; padding: 0; margin: 0 auto; width: 44px; border-radius: 999px; }
@@ -927,7 +939,14 @@ select optgroup { background-color: var(--bg-elev); color: var(--fg); }
  * pointermove would arrive after the scroll had already started, and the list
  * would stutter on the way past.
  */
-.msg-row { touch-action: pan-y; -webkit-touch-callout: none; }
+/*
+ * `pinch-zoom` is listed alongside `pan-y` rather than left implied: naming any
+ * value at all drops every gesture not named, so a bare `pan-y` also told the
+ * browser not to zoom here. Pinch worked on an open message and died on the
+ * list, which read as the zoom being broken at random. The swipe is unaffected
+ * -- horizontal is still not in the list, so it still arrives here.
+ */
+.msg-row { touch-action: pan-y pinch-zoom; -webkit-touch-callout: none; }
 .msg-row.swiping { z-index: 4; box-shadow: var(--shadow-2); }
 /* The row being dragged is the one thing on screen that must not lag. */
 .msg-row.swiping, .mail-list-pull { will-change: transform; }
@@ -1051,6 +1070,46 @@ select optgroup { background-color: var(--bg-elev); color: var(--fg); }
   .files-table .hide-mobile { display: none; }
   .event-popover { width: calc(100vw - 24px); }
 }
+/*
+ * Touch targets.
+ *
+ * `pointer: coarse` rather than a width, for the reason ui/misc.tsx gives about
+ * useIsTouch: width decides the layout, the pointer decides what has to be
+ * aimed at. A tablet in landscape is a wide screen with fat targets.
+ *
+ * Everything here was under the 44px a fingertip actually covers -- the top bar
+ * icons at 36px (the search filter at 30), the folder twisty at 18, the row ⋮
+ * at 24. Growing the boxes would reflow a top bar that has no room to give, so
+ * each control keeps the size it draws at and gains a transparent hit area
+ * centred on it. Rows grow for real, because a 44px hit area inside a 36px row
+ * would reach into the rows above and below and steal their taps.
+ */
+@media (pointer: coarse) {
+  .icon-btn, .nav-twisty { position: relative; }
+  /* `.drill-into` already draws at 44px and is excluded: expanding a control
+     that is big enough only lets it reach into its neighbour. */
+  .icon-btn:not(.drill-into)::after, .nav-twisty[role="button"]::after {
+    content: "";
+    position: absolute;
+    left: 50%;
+    top: 50%;
+    width: max(100%, 44px);
+    height: max(100%, 44px);
+    transform: translate(-50%, -50%);
+  }
+  /*
+   * The ⋮ is the exception, and narrower on purpose. It sits immediately left
+   * of the drill chevron, and at a full 44px wide the two hit areas overlapped
+   * by 4px -- so the right edge of "folder options" silently drilled instead.
+   * The row is 44px tall here, so it still gets the full height, which is the
+   * axis a thumb actually misses on.
+   */
+  .nav-item .nav-more::after { width: max(100%, 36px); }
+  .sidebar .nav-item, .cal-list-item { height: 44px; }
+  /* The hamburger is the one control in the corner a thumb is worst at, so it
+     gets real distance from the bezel rather than just a wider hit area. */
+  .topbar { padding-left: max(12px, env(safe-area-inset-left)); }
+}
 @media (hover: none) {
   .msg-row .msg-check { opacity: 1; }
   .msg-row .msg-actions { display: none !important; }