fix(web): floating preview shows a frame immediately on open + is resizable

The floating preview opened empty and stayed empty until the display next
CHANGED - the SSE stream only pushes frames on change, and the panel only
consumed frames while already open, so the connection's initial frame
(sent while the panel was closed) was dropped. Reported from mobile
testing as "the button doesn't work".

- updateDisplayPreview now caches the latest frame globally regardless of
  panel state; opening the panel populates the image from that cache
  instantly, then live frames take over.
- Resizable: a size button cycles 192/256/384/512px presets (persisted per
  browser; works on touch), and desktop additionally gets a native drag
  handle (CSS resize: both). The image is fluid within the panel; on
  phones the panel is capped to the viewport width. The size icon
  (fa-up-right-and-down-left-from-center) is verified present in the
  vendored FA 6.0.0.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEZK1P1Q1fu5pcuVrkrCFZ

web_interface/static/v3/app.css

@@ -1368,6 +1368,11 @@ button.bg-white {
     border-radius: 0.5rem;
     box-shadow: 0 8px 24px rgba(0, 0, 0, 0.4);
     overflow: hidden;
+    /* Desktop: draggable resize handle (bottom-left visually, since the
+       panel is anchored to the right). Touch devices use the size button. */
+    resize: both;
+    min-width: 160px;
+    max-width: 90vw;
 }
 .floating-preview img {
     background-color: #000;
@@ -1387,7 +1392,8 @@ button.bg-white {
     justify-content: center;
 }
 @media (max-width: 640px) {
-    .floating-preview img {
-        width: 192px !important;
+    /* Whatever size is chosen, never wider than the phone viewport */
+    .floating-preview {
+        max-width: calc(100vw - 2rem);
     }
 }

web_interface/static/v3/app.js

@@ -389,6 +389,29 @@ window.toggleFloatingPreview = function(open) {
     window.updateFloatingPreviewVisibility();
 };
 
+// Preset widths the size button cycles through (px). Desktop users can also
+// drag the panel's native resize handle (CSS resize: both).
+const FLOATING_PREVIEW_SIZES = [192, 256, 384, 512];
+
+window.applyFloatingPreviewSize = function() {
+    const panel = document.getElementById('floating-preview');
+    if (!panel) return;
+    let size = 256;
+    try { size = parseInt(localStorage.getItem('ledmatrix-floating-preview-size'), 10) || 256; } catch { /* no-op */ }
+    panel.style.width = size + 'px';
+    // Clear any manual drag-resize height so the image's aspect ratio rules
+    panel.style.height = '';
+};
+
+window.cycleFloatingPreviewSize = function() {
+    let size = 256;
+    try { size = parseInt(localStorage.getItem('ledmatrix-floating-preview-size'), 10) || 256; } catch { /* no-op */ }
+    const idx = FLOATING_PREVIEW_SIZES.indexOf(size);
+    const next = FLOATING_PREVIEW_SIZES[(idx + 1) % FLOATING_PREVIEW_SIZES.length];
+    try { localStorage.setItem('ledmatrix-floating-preview-size', String(next)); } catch { /* no-op */ }
+    window.applyFloatingPreviewSize();
+};
+
 window.updateFloatingPreviewVisibility = function(tab) {
     const panel = document.getElementById('floating-preview');
     const toggle = document.getElementById('floating-preview-toggle');
@@ -402,8 +425,19 @@ window.updateFloatingPreviewVisibility = function(tab) {
     const onOverview = active === 'overview';
     let open = false;
     try { open = localStorage.getItem('ledmatrix-floating-preview') === '1'; } catch { /* no-op */ }
-    panel.style.display = (!onOverview && open) ? 'block' : 'none';
+    const showPanel = !onOverview && open;
+    panel.style.display = showPanel ? 'block' : 'none';
     toggle.style.display = (!onOverview && !open) ? 'flex' : 'none';
+    if (showPanel) {
+        window.applyFloatingPreviewSize();
+        // Show the last cached frame immediately — SSE only pushes on
+        // display changes, so a freshly opened panel would otherwise stay
+        // empty until the next change.
+        const img = document.getElementById('floating-preview-img');
+        if (img && !img.src && window._lastPreviewFrame) {
+            img.src = 'data:image/png;base64,' + window._lastPreviewFrame;
+        }
+    }
 };
 
 document.addEventListener('DOMContentLoaded', function() {

web_interface/static/v3/js/app-shell.js

@@ -1878,6 +1878,13 @@
             const ledCanvas = document.getElementById('ledCanvas');
             const placeholder = document.getElementById('displayPlaceholder');
 
+            // Always cache the latest frame so the floating preview can show
+            // something the moment it's opened — SSE only sends frames when
+            // the display CHANGES, so a late subscriber would otherwise stare
+            // at an empty panel until the next change.
+            if (data.image) {
+                window._lastPreviewFrame = data.image;
+            }
             // Feed the floating mini preview (lives in base.html, present on
             // every tab) before the overview-only guard below.
             const floatImg = document.getElementById('floating-preview-img');

web_interface/templates/v3/base.html

@@ -563,13 +563,20 @@
         <div id="floating-preview" class="floating-preview" style="display:none">
             <div class="flex items-center justify-between px-2 py-1">
                 <span class="text-xs text-gray-300"><i class="fas fa-tv mr-1"></i>Live Preview</span>
+                <span class="flex items-center">
+                    <button type="button" onclick="window.cycleFloatingPreviewSize()"
+                            class="text-gray-400 hover:text-white ml-2" aria-label="Cycle preview size"
+                            title="Cycle preview size (drag the corner to resize on desktop)">
+                        <i class="fas fa-up-right-and-down-left-from-center text-xs"></i>
+                    </button>
                     <button type="button" onclick="window.toggleFloatingPreview(false)"
                             class="text-gray-400 hover:text-white ml-2" aria-label="Close live preview">
                         <i class="fas fa-times text-xs"></i>
                     </button>
+                </span>
             </div>
             <img id="floating-preview-img" alt="Live display preview"
-                 style="image-rendering: pixelated; display: block; width: 256px; height: auto;">
+                 style="image-rendering: pixelated; display: block; width: 100%; height: auto;">
         </div>
         <button id="floating-preview-toggle" type="button" onclick="window.toggleFloatingPreview(true)"
                 class="floating-preview-toggle" style="display:none"