fix(app): keep the window draggable by its tab bar while a dialog is open (#11766)

Co-authored-by: meanaverage <18336095+meanaverage@users.noreply.github.com>

app/src/global.scss

@@ -13,6 +13,18 @@ body {
     -webkit-app-region: no-drag;
 }
 
+// A dialog covers the whole window, tab bar included, so the window couldn't be moved while one was open: keep a
+// strip the height of the tab bar draggable. The dialog box itself is below it and stays no-drag.
+.modal::before {
+    content: '';
+    position: fixed;
+    top: 0;
+    left: 0;
+    right: 0;
+    height: calc(38px * var(--spaciness, 1));
+    -webkit-app-region: drag;
+}
+
 .selectable {
     user-select: text;
 }