fix: show resize overlay during dialog shell gutter resize

Apply the is-resizing class at resize start and remove it on end,
and raise the related resize overlay z-index so sibling content stays
visible underneath the resize indicator.

web/css/resolver-main.css

@@ -734,7 +734,7 @@
 .mr-missing-browser.is-resizing::after {
     content: "";
     position: absolute;
-    z-index: 1;
+    z-index: 9999;
     inset: 0;
     cursor: col-resize;
     pointer-events: auto;

web/resolver/shell/dialog_shell_methods.js

@@ -341,7 +341,11 @@ export const dialogShellMethods = {
         const pageX = Number(event?.pageX ?? event?.clientX);
         const pageY = Number(event?.pageY ?? event?.clientY);
         state.pendingArgs = args;
-        state.hasMoved = true;
+        if (!state.hasMoved) {
+            state.hasMoved = true;
+            state.gutter?.classList?.add('is-resizing');
+            state.gutter?.parentElement?.classList?.add('is-resizing');
+        }
 
         const offset = state.vertical
             ? pageY - state.appliedPageY
@@ -408,6 +412,8 @@ export const dialogShellMethods = {
         }
         state.gutter.style.transform = state.originalTransform;
         state.gutter.style.willChange = state.originalWillChange;
+        state.gutter?.classList?.remove('is-resizing');
+        state.gutter?.parentElement?.classList?.remove('is-resizing');
         document.removeEventListener('mouseup', state.mouseUpHandler, true);
         window.removeEventListener('blur', state.blurHandler);
         return true;