Ask RetroArch for the picture a mirrored state carries

README.md

@@ -541,8 +541,14 @@ Chrono Trigger (USA) [study-pc slot 3].state
 ```
 
 Naming the machine keeps two of them from overwriting each other's slots.
-RetroArch's automatic state is `[... auto]`, its `.bak` copies are skipped, and
-savestate thumbnails go up with the state if you have them switched on.
+RetroArch's automatic state is `[... auto]`, and its `.bak` copies are skipped.
+
+A state goes up with the picture RetroArch takes as it writes it. RetroArch
+takes one only when savestate thumbnails are on, and they are off by default,
+so the built-in RetroArch launch asks for them in its generated config whenever
+it is mirroring that launch's states. It asks only for that launch, and never
+asks for them to be turned off. A custom mapping gets no generated config, so
+there the picture goes up if you have thumbnails switched on yourself.
 
 Only the slots a run actually wrote are sent, and anything over 128 MiB is
 logged rather than sent: a state and the request framing it are both in memory

src/main/launcher.ts

@@ -825,6 +825,10 @@ export class Launcher {
               // config beats the redirect.
               saveDir: savePaths?.saveDir,
               stateDir: savePaths?.stateDir,
+              // On the same terms as the interval: the picture is only worth
+              // taking where there is a state mirror to carry it, and this is
+              // the same condition the reading below was taken under.
+              stateThumbnails: Boolean(savePaths) && syncsStates,
             })
           : null;
       // The interval is the one thing the arguments below do not show: they

src/main/saves/retroarch.test.ts

@@ -138,6 +138,36 @@ test("a launch pins the directories its saves and states belong in", () => {
   }
 });
 
+test("a launch mirroring its states asks for the pictures to mirror", () => {
+  // RetroArch writes the thumbnail beside a state only when the setting is on,
+  // and it is off by default, so a launch that says nothing leaves every state
+  // in RomM without its picture.
+  const written =
+    retroarchLaunchConfig({
+      autosaveSeconds: 0,
+      stateDir: "/data/4755/states",
+      stateThumbnails: true,
+    }) ?? "";
+
+  assert.match(written, /^savestate_thumbnail_enable = "true"$/m);
+});
+
+test("a launch not mirroring its states leaves the thumbnails alone", () => {
+  // Never written as "false": the shell has no use for the picture here, which
+  // is not a reason to take it away from a user who keeps thumbnails for
+  // RetroArch's own slot menu.
+  for (const stateThumbnails of [false, undefined]) {
+    const written =
+      retroarchLaunchConfig({
+        autosaveSeconds: 0,
+        stateDir: "/data/4755/states",
+        stateThumbnails,
+      }) ?? "";
+
+    assert.doesNotMatch(written, /savestate_thumbnail_enable/);
+  }
+});
+
 test("a directory a config cannot quote is left unsaid", () => {
   // A retroarch.cfg value is a quoted string with no escape for a quote inside
   // it, so the alternative to saying nothing is a config that does not parse.

src/main/saves/retroarch.ts

@@ -85,6 +85,9 @@ export interface LaunchSettings {
    *  directories to the user's own settings. */
   saveDir?: string;
   stateDir?: string;
+  /** Whether to ask for the picture RetroArch files beside a state. False and
+   *  undefined both ask for nothing, which leaves the user's setting alone. */
+  stateThumbnails?: boolean;
 }
 
 /**
@@ -154,6 +157,16 @@ export function retroarchLaunchConfig(settings: LaunchSettings): string | null {
   }
   lines.push(...saveDirectoryLines(settings));
 
+  // The picture a mirrored state carries in RomM is the one RetroArch takes as
+  // it writes the state, and it takes none unless asked: savestate thumbnails
+  // are off by default, so a launch that says nothing sends every state up
+  // bare. Only ever turned on, never off. A launch not mirroring its states
+  // has no use for the picture, and a user who keeps thumbnails for
+  // RetroArch's own slot menu keeps them.
+  if (settings.stateThumbnails) {
+    lines.push('savestate_thumbnail_enable = "true"');
+  }
+
   if (lines.length === 0) return null;
   return [...GENERATED_HEADER, ...lines, ""].join("\n");
 }

src/main/saves/states.ts

@@ -40,7 +40,9 @@ import { safeFileName, safeFileNameComponent } from "../safety.ts";
  */
 export const MAX_STATE_BYTES = 128 * 1024 * 1024;
 
-/** The picture RetroArch writes beside a state when thumbnails are on. */
+/** The picture RetroArch writes beside a state when thumbnails are on, which
+ *  is what the generated config asks for in `retroarch.ts`: the setting is off
+ *  by default, so without that a mirrored state never has one. */
 export const THUMBNAIL_SUFFIX = ".png";
 
 /** Whether a launch should mirror its states at all. */