diff --git a/src/app/shared/task-execution-viewer/task-execution-viewer.css b/src/app/shared/task-execution-viewer/task-execution-viewer.css
index 4965fef..df24a5d 100644
--- a/src/app/shared/task-execution-viewer/task-execution-viewer.css
+++ b/src/app/shared/task-execution-viewer/task-execution-viewer.css
@@ -968,3 +968,11 @@
   background: #f8fafc;
   padding: 6px 8px;
 }
+
+/*
+ * A tree grows and shrinks as its nodes are opened and closed. Sized to its content, the modal -
+ * centred on the page - would jump with every click; at a fixed height only its content scrolls.
+ */
+.execution-json-modal {
+  height: 90vh;
+}
diff --git a/src/app/shared/task-execution-viewer/task-execution-viewer.html b/src/app/shared/task-execution-viewer/task-execution-viewer.html
index b816c8f..c1bf5b1 100644
--- a/src/app/shared/task-execution-viewer/task-execution-viewer.html
+++ b/src/app/shared/task-execution-viewer/task-execution-viewer.html
@@ -653,7 +653,8 @@
   @if (outputPreviewModal(); as outputModal) {
   <div class="fixed inset-0 z-9999 flex items-center justify-center p-4">
     <div class="absolute inset-0 bg-black/45" (click)="closeOutputPreview($event)"></div>
-    <div class="relative z-10 flex max-h-[90vh] w-full max-w-4xl flex-col overflow-hidden rounded-xl border border-slate-200 bg-white shadow-2xl">
+    <div class="relative z-10 flex max-h-[90vh] w-full max-w-4xl flex-col overflow-hidden rounded-xl border border-slate-200 bg-white shadow-2xl"
+      [class.execution-json-modal]="outputModal.json">
       <div class="flex items-start justify-between gap-4 border-b border-slate-200 px-5 py-4">
         <div class="min-w-0">
           <h3 class="text-lg font-semibold text-slate-900">{{ outputPreviewTitle(outputModal) }}</h3>
@@ -674,7 +675,8 @@
   @if (intermediateInputPreviewModal(); as inputModal) {
   <div class="fixed inset-0 z-9999 flex items-center justify-center p-4">
     <div class="absolute inset-0 bg-black/45" (click)="closeIntermediateInputPreview($event)"></div>
-    <div class="relative z-10 flex max-h-[90vh] w-full max-w-4xl flex-col overflow-hidden rounded-xl border border-slate-200 bg-white shadow-2xl">
+    <div class="relative z-10 flex max-h-[90vh] w-full max-w-4xl flex-col overflow-hidden rounded-xl border border-slate-200 bg-white shadow-2xl"
+      [class.execution-json-modal]="inputModal.json">
       <div class="flex items-start justify-between gap-4 border-b border-slate-200 px-5 py-4">
         <div class="min-w-0">
           <h3 class="text-lg font-semibold text-slate-900">{{ intermediateInputPreviewTitle(inputModal) }}</h3>