Draw signatures on a phone via QR code in the Sign tool (#7335)

# Description of Changes

Scan a QR code in the Sign tool, draw your signature on your phone, and
it appears on your desktop ready to place. Rides the mobile scanner's
existing transfer sessions — no new backend endpoints.

**Desktop:** a **Mobile upload** button above the signature source
selector shows a QR code. When the signature arrives, the modal closes,
it lands in the matching source, and **placement activates
automatically** — click the PDF to place.

**Phone:** a new public `/mobile-sign` page with three tabs (same order
as the desktop sources):

- **Draw** → canvas signature. Touch-first pad (pointer events,
DPR-aware, smoothed strokes, undo/clear, black/blue ink, 3 pen sizes),
exported as a transparent PNG cropped to the ink. Compact layout in
phone landscape.
- **Photo** → image signature. "Take a photo" opens the camera directly;
"From gallery" opens the picker. A preview of the current image
signature now shows in the desktop's Image source (previously arrival
was invisible until placement — also fixes this for saved image
signatures).
- **Type** → text signature. Travels as data (text + font + colour), so
it stays *editable* on the desktop. Fonts are the sign tool's own
text-mode list.

**Security:** the transfer endpoints are unauthenticated by design
(10-min sessions, files deleted after download — same model as the
scanner). The desktop treats every arrival as untrusted: images only,
and the text payload is clamped field by field.

**Config:** new `system.enableMobileSignature` flag (default on),
independent of `enableMobileScanner`; the shared endpoints accept
either. The Tauri desktop app serves a self-contained `mobile-sign.html`
(draw-only), mirroring `mobile-upload.html`.

**Refactor:** the session lifecycle (create/poll/download/expiry) moved
out of `MobileUploadModal` into a shared `useMobileTransferSession`
hook; the scanner modal now uses it, behaviour unchanged.

Also fixes two bugs hit along the way: the signature pad collapsing to
its 150px intrinsic height (indefinite parent height), and a setState
loop in `SignSettings` when text parameters are set programmatically
(draft-sync effects ping-ponging).

## Screenshots

| Desktop: QR entry | Phone: draw | Desktop: received |
|---|---|---|
| ![QR
modal](https://raw.githubusercontent.com/Stirling-Tools/Stirling-PDF/pr-7335-assets/shot-1-qr-modal.png)
| ![Phone draw
tab](https://raw.githubusercontent.com/Stirling-Tools/Stirling-PDF/pr-7335-assets/shot-2-phone-draw.png)
| ![Signature
received](https://raw.githubusercontent.com/Stirling-Tools/Stirling-PDF/pr-7335-assets/shot-3-desktop-received.png)
|

## How to test

1. Open the app on an address your phone can reach (not `localhost`),
Sign tool → **Mobile upload**, scan the QR.
2. Draw → **Send to computer** → it becomes the active canvas signature
and placement is live: click the PDF to place.
3. Photo tab → arrives in the Image source with a preview. Type tab →
arrives editable in the Text source.
4. Flags: `enableMobileSignature: false` hides the button; signature
still works with the scanner disabled.

Verified end-to-end (all three kinds, portrait/landscape/tablet) plus
`task frontend:check` and the touched backend tests.

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [x] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [x] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [x] I have run `task check` to verify linters, typechecks, and tests
pass
- [x] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

Author: reecebrowne

app/common/src/main/java/stirling/software/common/model/ApplicationProperties.java

@@ -1046,6 +1046,8 @@ public static class System {
 
         // 'https://app.example.com'). If not set, falls back to backendUrl.
         private boolean enableMobileScanner = true; // Enable mobile phone QR code upload feature
+        private boolean enableMobileSignature =
+                true; // Enable drawing signatures on a phone via QR code
         private MobileScannerSettings mobileScannerSettings = new MobileScannerSettings();
         private ServerCertificate serverCertificate = new ServerCertificate();
 

app/common/src/main/java/stirling/software/common/util/RequestUriUtils.java

@@ -56,8 +56,10 @@ public static boolean isStaticResource(String contextPath, String requestURI) {
             return true;
         }
 
-        // Mobile scanner page for QR code-based file uploads (peer-to-peer, no backend auth needed)
-        if (normalizedUri.startsWith("/mobile-scanner")) {
+        // Mobile pages reached by scanning a QR code (peer-to-peer, no backend auth
+        // needed): /mobile-scanner uploads photos, /mobile-sign draws a signature.
+        if (normalizedUri.startsWith("/mobile-scanner")
+                || normalizedUri.startsWith("/mobile-sign")) {
             return true;
         }
 

app/common/src/test/java/stirling/software/common/util/RequestUriUtilsTest.java

@@ -73,6 +73,13 @@ void testIsStaticResource_mobileScannerPath() {
         assertTrue(RequestUriUtils.isStaticResource("/mobile-scanner"));
     }
 
+    @Test
+    void testIsStaticResource_mobileSignPath() {
+        // The phone-side signature drawing page, reached from the Sign tool QR code.
+        assertTrue(RequestUriUtils.isStaticResource("/mobile-sign"));
+        assertTrue(RequestUriUtils.isStaticResource("/app", "/app/mobile-sign"));
+    }
+
     @Test
     void testIsStaticResource_portalShell() {
         // The admin portal SPA shell (/processor) is served pre-auth so it's directly navigable.

app/core/build.gradle

@@ -312,8 +312,9 @@ tasks.register('cleanFrontendAssets', Delete) {
     delete generatedFrontendPaths.collect { new File(resourcesStaticDir, it) }
     // Prerendered per-route SPA pages (e.g. compress.html) carry per-tool OG tags and are
     // copied from the frontend build. Remove stale ones so renamed/removed tools don't linger.
-    // api-landing.html and mobile-upload.html are real backend source files, not generated artifacts.
-    delete fileTree(dir: resourcesStaticDir, includes: ['*.html'], excludes: ['api-landing.html', 'mobile-upload.html'])
+    // api-landing.html, mobile-upload.html and mobile-sign.html are real backend source files,
+    // not generated artifacts.
+    delete fileTree(dir: resourcesStaticDir, includes: ['*.html'], excludes: ['api-landing.html', 'mobile-upload.html', 'mobile-sign.html'])
     // Nested prerendered route pages (e.g. settings/people.html)
     delete new File(resourcesStaticDir, 'settings')
 }

app/core/src/main/java/stirling/software/SPDF/controller/api/misc/ConfigController.java

@@ -194,6 +194,9 @@ public ResponseEntity<Map<String, Object>> getAppConfig(HttpServletRequest reque
             configData.put(
                     "enableMobileScanner",
                     applicationProperties.getSystem().isEnableMobileScanner());
+            configData.put(
+                    "enableMobileSignature",
+                    applicationProperties.getSystem().isEnableMobileSignature());
             configData.put(
                     "mobileScannerConvertToPdf",
                     applicationProperties.getSystem().getMobileScannerSettings().isConvertToPdf());

app/core/src/main/java/stirling/software/SPDF/controller/api/misc/MobileScannerController.java

@@ -62,12 +62,17 @@ public MobileScannerController(
     }
 
     /**
-     * Check if mobile scanner feature is enabled
+     * Check if any feature backed by these transfer sessions is enabled. The mobile scanner and
+     * mobile signature drawing share this session/upload API, so the endpoints stay available while
+     * either feature is on; each flag independently controls only its own UI.
      *
      * @return Error response if disabled, null if enabled
      */
     private ResponseEntity<Map<String, Object>> checkFeatureEnabled() {
-        if (!applicationProperties.getSystem().isEnableMobileScanner()) {
+        boolean anyEnabled =
+                applicationProperties.getSystem().isEnableMobileScanner()
+                        || applicationProperties.getSystem().isEnableMobileSignature();
+        if (!anyEnabled) {
             return ResponseEntity.status(HttpStatus.FORBIDDEN)
                     .body(
                             Map.of(
@@ -275,7 +280,8 @@ public ResponseEntity<Resource> downloadFile(
             @Parameter(description = "Filename to download", required = true) @PathVariable
                     String filename) {
 
-        if (!applicationProperties.getSystem().isEnableMobileScanner()) {
+        if (!applicationProperties.getSystem().isEnableMobileScanner()
+                && !applicationProperties.getSystem().isEnableMobileSignature()) {
             return ResponseEntity.status(HttpStatus.FORBIDDEN).build();
         }
 

app/core/src/main/java/stirling/software/SPDF/controller/web/ReactRoutingController.java

@@ -80,6 +80,8 @@ public class ReactRoutingController {
     private boolean saasLandingExists = false;
     private String cachedMobileUploadHtml;
     private boolean mobileUploadHtmlExists = false;
+    private String cachedMobileSignHtml;
+    private boolean mobileSignHtmlExists = false;
 
     @PostConstruct
     public void init() {
@@ -103,10 +105,12 @@ public void init() {
         }
 
         // Desktop (Tauri) serves the SPA from its bundled webview, so a phone scanning the QR can't
-        // load the React /mobile-scanner route from the local backend. Cache the self-contained
-        // static upload page to serve at that route in desktop mode instead.
+        // load the React /mobile-scanner or /mobile-sign routes from the local backend. Cache the
+        // self-contained static pages to serve at those routes in desktop mode instead.
         this.cachedMobileUploadHtml = readStaticHtml("mobile-upload.html");
         this.mobileUploadHtmlExists = this.cachedMobileUploadHtml != null;
+        this.cachedMobileSignHtml = readStaticHtml("mobile-sign.html");
+        this.mobileSignHtmlExists = this.cachedMobileSignHtml != null;
 
         // Check for external index.html first (customFiles/static/)
         Path externalIndexPath = Path.of(InstallationPathConfig.getStaticPath(), "index.html");
@@ -268,6 +272,17 @@ public ResponseEntity<String> serveMobileScanner(HttpServletRequest request) {
         return serveIndexHtml(request);
     }
 
+    @GetMapping(value = "/mobile-sign", produces = MediaType.TEXT_HTML_VALUE)
+    public ResponseEntity<String> serveMobileSign(HttpServletRequest request) {
+        if (isDesktopMode() && mobileSignHtmlExists) {
+            return ResponseEntity.ok()
+                    .cacheControl(CacheControl.noCache().mustRevalidate())
+                    .contentType(MediaType.TEXT_HTML)
+                    .body(cachedMobileSignHtml);
+        }
+        return serveIndexHtml(request);
+    }
+
     @GetMapping(value = "/auth/callback/tauri", produces = MediaType.TEXT_HTML_VALUE)
     public ResponseEntity<String> serveTauriAuthCallback(HttpServletRequest request) {
         // cachedCallbackHtml is always initialized in @PostConstruct

app/core/src/main/resources/settings.yml.template

@@ -188,6 +188,7 @@ system:
   backendUrl: "" # Backend base URL for SAML/OAuth/API callbacks (e.g. 'http://localhost:8080' for dev, 'https://api.example.com' for production). REQUIRED for SSO authentication to work correctly. This is where your IdP will send SAML responses and OAuth callbacks. Leave empty to default to 'http://localhost:8080' in development.
   frontendUrl: "" # Frontend URL for invite email links (e.g. 'https://app.example.com'). Optional - if not set, will use backendUrl. This is the URL users click in invite emails.
   enableMobileScanner: true # Enable mobile phone QR code upload feature. Requires frontendUrl to be configured.
+  enableMobileSignature: true # Enable drawing signatures on a phone via QR code from the Sign tool. Requires frontendUrl to be configured.
   mobileScannerSettings:
     convertToPdf: true # Automatically convert uploaded images to PDF format. If false, images are kept as-is.
     imageResolution: full # Image resolution for mobile uploads: 'full' (original size) or 'reduced' (max 1200px on longest side). Only applies when convertToPdf is true.