perf: use native composer sizing where supported (#6760)

Use native CSS field-sizing for composer auto-grow where supported, keeping the JS path as fallback. Includes a fallback-parity fix so an empty composer holds its resting height instead of measuring the placeholder.

Co-authored-by: starship-s <starship-s@users.noreply.github.com>

static/messages.js

@@ -7236,7 +7236,28 @@ function autoResize(){
   }
   const el=$('msg');
   const _nextValue=String(el.value||'');
+  if(typeof CSS!=='undefined'&&typeof CSS.supports==='function'&&CSS.supports('field-sizing','content')){
+    if(el.style.height) el.style.height='';
+    _composerLastResizeValue=_nextValue;
+    updateSendBtn();
+    return;
+  }
   const _isAppendOnly=_nextValue.length>_composerLastResizeValue.length&&_nextValue.startsWith(_composerLastResizeValue);
+  // An EMPTY composer has no content to measure, so clear any inline height and
+  // let the CSS `min-height` define the resting size. Measuring instead would
+  // read the PLACEHOLDER's scrollHeight — a long busy/compression hint wraps to
+  // two or three lines and would grow the empty composer (71px for the English
+  // busy hint, 97px for the French compression one) purely because of hint text.
+  // That made the empty height history-dependent on this path: 44px on a fresh
+  // send, but grown after any later resize while empty. The native
+  // `field-sizing` path above always holds the resting height, so clearing here
+  // keeps both paths on the same contract.
+  if(!_nextValue){
+    if(el.style.height) el.style.height='';
+    _composerLastResizeValue=_nextValue;
+    updateSendBtn();
+    return;
+  }
   const _fitsCurrentHeight=el.scrollHeight<=el.offsetHeight;
   // Only a direct append at the natural one-row height can skip the height
   // round trip. Replacements and an already-tall composer must remeasure so the