Let the toggle come back to the theme you were on

The top-bar toggle went to light from anything dark, and back to plain
"dark" -- which quietly moved an ihasmail user onto a theme they had
never chosen, two clicks and no way to tell what had happened. It did
the same to "match system", which the toggle could not restore at all;
the comment above it conceded as much and sent people to Settings.

There is more than one way to be dark now, so the way back is
remembered: lastDarkTheme holds whichever non-light theme was last
chosen, and the toggle returns to that.

The remembering lives in update(), the single path every way of setting
a theme goes through -- the toggle, Appearance, an imported settings
file -- so a fourth way to choose one cannot forget to record it. Light
never overwrites it, since light is the side being toggled away from.

The button's label follows: "Switch to the ihasmail theme", "Switch to
your system theme", rather than claiming everything dark is "dark mode".

Confirmed in a browser, not only in tests: from a fresh profile the
round trip ihasmail -> light -> ihasmail returns to ihasmail, and
system -> light -> system returns to system, with the label naming the
destination each time.

web/src/lib/__tests__/theme.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { DEFAULT_SETTINGS, isDarkTheme, type Theme } from "@/store/settings";
+import { DEFAULT_SETTINGS, isDarkTheme, toggleTarget, useSettings, type Theme } from "@/store/settings";
 import { loadJson, saveJson } from "@/lib/storage";
 
 /**
@@ -90,3 +90,57 @@ describe("the default theme", () => {
     });
   });
 });
+
+describe("the top-bar toggle", () => {
+  it("goes to light from anything dark", () => {
+    expect(toggleTarget("dark", "ihasmail")).toBe("light");
+    expect(toggleTarget("dark", "dark")).toBe("light");
+    expect(toggleTarget("dark", "system")).toBe("light");
+  });
+
+  it("comes back to the theme you were actually on", () => {
+    // The whole point: two clicks from ihasmail must return to ihasmail, not
+    // deposit you on plain dark.
+    expect(toggleTarget("light", "ihasmail")).toBe("ihasmail");
+    expect(toggleTarget("light", "dark")).toBe("dark");
+  });
+
+  it("can bring back \"match system\", which the toggle used to strand", () => {
+    expect(toggleTarget("light", "system")).toBe("system");
+  });
+
+  it("round-trips every dark theme there is", () => {
+    for (const t of ["dark", "ihasmail", "system"] as const) {
+      expect(toggleTarget(toggleTarget("light", t) === "light" ? "light" : "dark", t), t).toBe("light");
+      expect(toggleTarget("light", t), t).toBe(t);
+    }
+  });
+});
+
+describe("remembering which dark theme you were on", () => {
+  const setTheme = (t: Theme) => {
+    useSettings.getState().update({ theme: t });
+    return useSettings.getState().settings;
+  };
+
+  it("records a dark theme chosen from Settings, not just from the toggle", () => {
+    // update() is the single path every way of choosing a theme goes through,
+    // which is why the remembering lives there rather than at the call sites.
+    expect(setTheme("dark").lastDarkTheme).toBe("dark");
+    expect(setTheme("ihasmail").lastDarkTheme).toBe("ihasmail");
+    expect(setTheme("system").lastDarkTheme).toBe("system");
+  });
+
+  it("does not let light overwrite it — that is the theme being toggled away from", () => {
+    setTheme("ihasmail");
+    expect(setTheme("light").lastDarkTheme).toBe("ihasmail");
+  });
+
+  it("survives a there-and-back through the toggle", () => {
+    setTheme("ihasmail");
+    const away = setTheme(toggleTarget("dark", useSettings.getState().settings.lastDarkTheme));
+    expect(away.theme).toBe("light");
+    const back = setTheme(toggleTarget("light", away.lastDarkTheme));
+    expect(back.theme).toBe("ihasmail");
+  });
+});

web/src/store/settings.ts

@@ -88,6 +88,14 @@ export interface Settings {
   eventCategories: Array<{ name: string; color: string }>;
   /** Default sending identity per account (JMAP has no such flag). */
   defaultIdentityByAccount: Record<string, string>;
+  /**
+   * The theme the top-bar toggle goes back to from light. Remembered rather
+   * than assumed, so flipping to light and back returns you to the theme you
+   * were on — "ihasmail", "system" or plain "dark" — instead of dropping
+   * everyone onto the same one. Never "light": that is the side being
+   * toggled away from.
+   */
+  lastDarkTheme: Exclude<Theme, "light">;
 }
 
 export const DEFAULT_SETTINGS: Settings = {
@@ -152,6 +160,7 @@ export const DEFAULT_SETTINGS: Settings = {
     { name: "Family", color: "#9333ea" },
   ],
   defaultIdentityByAccount: {},
+  lastDarkTheme: "ihasmail",
 };
 
 /**
@@ -215,14 +224,18 @@ applyDateTimePrefs(initialSettings);
 export const useSettings = create<SettingsState>((set, get) => ({
   settings: initialSettings,
   update(patch) {
-    const settings = { ...get().settings, ...patch };
+    // Picking a theme anywhere — the toggle, Appearance, an imported file —
+    // is what teaches the toggle where to come back to. Doing it here rather
+    // than at the call sites means a fourth way to set a theme cannot forget.
+    const next = patch.theme && patch.theme !== "light" ? { ...patch, lastDarkTheme: patch.theme } : patch;
+    const settings = { ...get().settings, ...next };
     saveJson("settings", settings);
     set({ settings });
     applyTheme(settings);
     applyDateTimePrefs(settings);
     // Dragging a splitter changes a device key on every frame and must not put
     // a request in the air; anything else is queued and coalesced.
-    if (Object.keys(patch).some((k) => !DEVICE_KEYS.has(k as keyof Settings))) {
+    if (Object.keys(next).some((k) => !DEVICE_KEYS.has(k as keyof Settings))) {
       queueSettingsPush(syncedPart(settings));
     }
   },
@@ -278,6 +291,15 @@ export function applyTheme(s: Settings = useSettings.getState().settings): void
   if (meta) meta.content = s.theme === "ihasmail" ? THEME_COLOR.ihasmail : dark ? THEME_COLOR.dark : THEME_COLOR.light;
 }
 
+/**
+ * Where the top-bar toggle goes next. Away from dark is always light; back
+ * from light is wherever you last were, which is the whole point of
+ * remembering it.
+ */
+export function toggleTarget(effective: "light" | "dark", lastDarkTheme: Settings["lastDarkTheme"]): Theme {
+  return effective === "dark" ? "light" : lastDarkTheme;
+}
+
 /** Whether a theme paints dark, resolving "system" against the OS. */
 export function isDarkTheme(theme: Theme, prefersDark = false): boolean {
   return theme === "dark" || theme === "ihasmail" || (theme === "system" && prefersDark);

web/src/views/AppShell.tsx

@@ -2,7 +2,7 @@ import { useEffect, useState, type ReactNode } from "react";
 import { Link, useLocation } from "wouter";
 import { Calendar, ChevronsUpDown, FolderOpen, HelpCircle, Mail, Menu as MenuIcon, Moon, PenSquare, Settings, Sun, Users, LogOut, Plus, RefreshCw } from "lucide-react";
 import { useSession } from "@/store/session";
-import { useEffectiveTheme, useSettings } from "@/store/settings";
+import { toggleTarget, useEffectiveTheme, useSettings } from "@/store/settings";
 import { useMail } from "@/store/mail";
 import { draftFromMailto, useCompose } from "@/store/compose";
 import { Avatar, useIsMobile } from "@/ui/misc";
@@ -205,22 +205,31 @@ function QuotaBar() {
 }
 
 /**
- * Flip between light and dark from the top bar.
+ * Flip to light and back from the top bar.
  *
- * The stored setting has a third value, "system", so the button acts on what
- * is actually on screen rather than on the setting: whichever theme you can
- * see, one click gives you the other one. Choosing "match system" again lives
- * in Settings › Appearance, where the three-way choice belongs.
+ * The setting has four values and only two of them are "light", so the button
+ * acts on what is actually on screen rather than on the setting: if you can
+ * see a dark theme, one click gives you light.
+ *
+ * Coming back is the part that needs remembering. There is more than one way
+ * to be dark — "dark", "ihasmail", or "system" while the OS is — so the way
+ * back is whichever you were on, kept in `lastDarkTheme`, rather than plain
+ * "dark" for everyone. Without that, two clicks would quietly move an
+ * ihasmail user onto a theme they never chose.
  */
 function ThemeToggle() {
   const effective = useEffectiveTheme();
+  const lastDarkTheme = useSettings((s) => s.settings.lastDarkTheme);
   const update = useSettings((s) => s.update);
-  const next = effective === "dark" ? "light" : "dark";
+  const next = toggleTarget(effective, lastDarkTheme);
+  // The label names where you are going, and going back is not always "dark"
+  // any more -- it is whichever theme you were on before flipping to light.
+  const label = next === "light" ? "light mode" : next === "system" ? "your system theme" : next === "ihasmail" ? "the ihasmail theme" : "dark mode";
   return (
     <button
       className="icon-btn"
-      aria-label={`Switch to ${next} mode`}
-      title={`Switch to ${next} mode`}
+      aria-label={`Switch to ${label}`}
+      title={`Switch to ${label}`}
       onClick={() => update({ theme: next })}
     >
       {effective === "dark" ? <Sun size={21} /> : <Moon size={21} />}