The action bar leaves the scroller: a flex footer, not a sticky trick

assets/css/builder.css

@@ -2696,32 +2696,39 @@ os-checkbox-label.atfb-check {
 	   button somewhere nobody could reach. Absolute against the body's
 	   `position: relative` is exact at every window size.
 
-	   No bottom padding: the sticky action bar below owns the bottom edge,
-	   and padding under it would be 14px the bar could never cover. */
+	   A flex column of exactly two things — the scroll region and the action
+	   bar — rather than one scroller with a sticky bar inside it. Sticky kept
+	   the bar in the scroll math, which produced both of its failure modes: a
+	   window a few pixels shorter than the content scrolled by those few
+	   pixels with nothing meaningful hidden, and a window taller than the
+	   content left the bar floating at the end of the text instead of on the
+	   window's edge. As a flex footer the bar is chrome: always on the bottom
+	   edge, never part of what scrolls. */
 	position: absolute;
 	inset: 0;
+	display: flex;
+	flex-direction: column;
 	inline-size: auto;
 	max-block-size: none;
 	block-size: auto;
-	padding: 14px 16px 0;
-	overflow-y: auto;
-	display: block;
+	padding: 0;
+	overflow: hidden;
 }
 
-/* Save and Cancel ride the bottom of the scrollport rather than the bottom of
-   the content: in a window the person sizes themselves, "scroll to find Save"
-   is a chore the chrome can simply remove. Stretched across the host's padding
-   and given a top border + opaque background so content sliding under it reads
-   as under it.
+/* The scrollable half. `min-block-size: 0` is what lets a flex child actually
+   shrink below its content and scroll, instead of stretching its parent. */
+.atfb-valwin__scroll {
+	flex: 1 1 auto;
+	min-block-size: 0;
+	overflow-y: auto;
+	padding: 14px 16px;
+}
 
-   `inset-block-end: 0`, exactly — a negative offset here parks part of the bar
-   past the scrollport edge, where the window body's `overflow: hidden` cuts
-   it in half. */
+/* The fixed half: Save and Cancel on the window's bottom edge, top border and
+   opaque background so content scrolling beneath reads as beneath. */
 .atfb-valwin--window .atfb-modal__actions {
-	position: sticky;
-	inset-block-end: 0;
-	margin-inline: -16px;
-	margin-block-start: 12px;
+	flex: 0 0 auto;
+	margin: 0;
 	padding: 10px 16px;
 	background: var( --atfb-bg );
 	border-block-start: 1px solid var( --atfb-border );

assets/js/builder.js

@@ -3102,7 +3102,13 @@ var allTerrainFormsBuilder = function(exports) {
       autofocus: ".atfb-valwin__pane:not([hidden]) input, .atfb-valwin__pane:not([hidden]) textarea",
       render: (body) => {
         const host = el("div", { class: "atfa atfb-valwin atfb-valwin--window" });
-        buildEditor(host, options, state, close, "window");
+        const scroll = el("div", { class: "atfb-valwin__scroll" });
+        host.append(scroll);
+        buildEditor(scroll, options, state, close, "window");
+        const actions = scroll.querySelector(".atfb-modal__actions");
+        if (actions) {
+          host.append(actions);
+        }
         body.append(host);
       },
       // Fires however the window closes — Save, Cancel, the title-bar X,

src/validation-editor.ts

@@ -397,8 +397,22 @@ function openAsChildWindow( manager: ShellWindowManager, parentId: string, optio
 		autofocus: '.atfb-valwin__pane:not([hidden]) input, .atfb-valwin__pane:not([hidden]) textarea',
 		render: ( body ) => {
 			const host = el( 'div', { class: 'atfa atfb-valwin atfb-valwin--window' } );
+			const scroll = el( 'div', { class: 'atfb-valwin__scroll' } );
+
+			host.append( scroll );
+			buildEditor( scroll, options, state, close, 'window' );
+
+			// The action bar is chrome, not content. Out of the scroller and
+			// into the host's flex column, it sits on the window's bottom
+			// edge at every size — and stops counting toward the scroll
+			// height, so a window tall enough to show everything scrolls by
+			// exactly nothing.
+			const actions = scroll.querySelector( '.atfb-modal__actions' );
+
+			if ( actions ) {
+				host.append( actions );
+			}
 
-			buildEditor( host, options, state, close, 'window' );
 			body.append( host );
 		},
 		// Fires however the window closes — Save, Cancel, the title-bar X,