fix: lay out the reader's pages at the pane's width so a tablet pane is not left empty

src/components/__tests__/reader-pager-width.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, it } from 'vitest';
+import fs from 'node:fs';
+import path from 'node:path';
+
+// Tablet, landscape: the first message opened after a relaunch and a
+// rotation showed an empty reading pane (toolbar and bottom bar only). The
+// pager laid its pages out at the window's width before it knew the pane's,
+// so initialScrollIndex put the open message at the wrong offset, and
+// removeClippedSubviews could leave that page detached. The pages now wait
+// for the measured width. No React Native renderer in the test env, so this
+// checks the wiring.
+
+const src = fs.readFileSync(path.resolve(__dirname, '..', '..', 'screens/EmailThreadScreen.tsx'), 'utf8');
+
+describe('reader pager width', () => {
+  it('renders the pages only once the pager is measured', () => {
+    expect(src).toMatch(/<View style=\{styles\.pagerViewport\} onLayout=\{onPagerLayout\}>\s+\{measuredWidth > 0 && \(\s+<FlatList/);
+    expect(src).toMatch(/const pageWidth = measuredWidth \|\| windowWidth;/);
+  });
+
+  it('does not clip the pages away', () => {
+    const pager = src.slice(src.indexOf('<FlatList'), src.indexOf('renderItem={({ item, index })'));
+    expect(pager).not.toMatch(/^\s+removeClippedSubviews\s*$/m);
+  });
+
+  it('re-renders the pages when their width changes', () => {
+    expect(src).toMatch(/\(\) => \[cacheVersion, neighboursReady, pageWidth\]/);
+  });
+});

src/screens/EmailThreadScreen.tsx

@@ -119,8 +119,13 @@ function EmailViewer({ route, navigation, embedded = false, onActiveChange }: Pr
   const { width: windowWidth } = useWindowDimensions();
   // Pages are as wide as the pager itself, which is the window on a phone
   // but only the detail pane in a split view. Measured, so a rotation or a
-  // pane resize re-sizes them.
+  // pane resize re-sizes them. The pages render only once it is measured:
-  const [pageWidth, setPageWidth] = React.useState(windowWidth);
+  // laid out at the window's width first, a split view's pager scrolled the
+  // opened message to the wrong offset, and on a tablet the pane once stayed
+  // empty (just the toolbar and the bottom bar) until the message was
+  // reopened.
+  const [measuredWidth, setMeasuredWidth] = React.useState(0);
+  const pageWidth = measuredWidth || windowWidth;
   const markRead = useEmailStore((s) => s.markRead);
   const setKeywordForEmails = useEmailStore((s) => s.setKeywordForEmails);
   const deleteEmail = useEmailStore((s) => s.deleteEmail);
@@ -336,12 +341,12 @@ function EmailViewer({ route, navigation, embedded = false, onActiveChange }: Pr
   }, [pageWidth]);
   const onPagerLayout = React.useCallback((e: LayoutChangeEvent) => {
     const width = Math.round(e.nativeEvent.layout.width);
-    if (width > 0) setPageWidth((prev) => (prev === width ? prev : width));
+    if (width > 0) setMeasuredWidth((prev) => (prev === width ? prev : width));
   }, []);
 
   const pagerExtraData = React.useMemo(
-    () => [cacheVersion, neighboursReady],
+    () => [cacheVersion, neighboursReady, pageWidth],
-    [cacheVersion, neighboursReady],
+    [cacheVersion, neighboursReady, pageWidth],
   );
 
   // While the body is pinch-zoomed the pager must not treat horizontal
@@ -815,10 +820,11 @@ function EmailViewer({ route, navigation, embedded = false, onActiveChange }: Pr
         </View>
       ) : (
         <>
+          <View style={styles.pagerViewport} onLayout={onPagerLayout}>
+          {measuredWidth > 0 && (
           <FlatList
             ref={listRef}
             style={styles.pagerViewport}
-            onLayout={onPagerLayout}
             onContentSizeChange={snapToActive}
             data={emails}
             extraData={pagerExtraData}
@@ -831,7 +837,8 @@ function EmailViewer({ route, navigation, embedded = false, onActiveChange }: Pr
             windowSize={3}
             initialNumToRender={1}
             maxToRenderPerBatch={2}
-            removeClippedSubviews
+            // Not removeClippedSubviews: with three pages it saves nothing,
+            // and Android can leave the open page detached after a resize.
             scrollEnabled={!pagerLocked && kbObstruction === 0}
             onScrollBeginDrag={releaseNeighbours}
             onMomentumScrollEnd={onMomentumEnd}
@@ -876,6 +883,8 @@ function EmailViewer({ route, navigation, embedded = false, onActiveChange }: Pr
               </View>
             )}
           />
+          )}
+          </View>
 
           {/* Bottom action bar (hidden while keyboard is up so quick reply stays visible) */}
           {kbObstruction === 0 && (