Fix crash loop: WebView touched off main thread during extraction cancellation

Root-caused via emulator stress test (rapid swiping through many articles):
withTimeoutOrNull's cancellation, and LaunchedEffect disposal when a page is
swiped away, both invoke invokeOnCancellation on kotlinx.coroutines.DefaultExecutor
rather than the calling coroutine's Main dispatcher. The handler called
webView.stopLoading()/destroy() directly, crashing with "A WebView method was
called on thread ..." on essentially every session once enough thin-content
articles were swiped past mid-extraction -- the reported crash loop. Now hops
back to the main looper before touching the WebView, and guards against a
double destroy() race between the JS-callback and cancellation paths.

Also fixed a real (non-crashing) leak: the visible reader WebView in
ReaderScreen was never destroy()-ed when its page left the pager, via
AndroidView's onRelease.

Reproduced pre-fix in ~80 rapid swipes on an emulator; post-fix, 320+ swipes
with real 107-feed data ran clean with stable native heap.

2 files changed, 34 insertions, 2 deletions

diff --git a/app/src/main/java/org/terst/feedreader/readability/ReaderExtractor.kt b/app/src/main/java/org/terst/feedreader/readability/ReaderExtractor.kt
index 9be08b0..286588a 100644
--- a/app/src/main/java/org/terst/feedreader/readability/ReaderExtractor.kt
+++ b/app/src/main/java/org/terst/feedreader/readability/ReaderExtractor.kt
@@ -2,6 +2,8 @@ package org.terst.feedreader.readability
 
 import android.annotation.SuppressLint
 import android.content.Context
+import android.os.Handler
+import android.os.Looper
 import android.webkit.WebView
 import android.webkit.WebViewClient
 import kotlinx.coroutines.Dispatchers
@@ -46,7 +48,16 @@ object ReaderExtractor {
             val webView = WebView(context)
             webView.settings.javaScriptEnabled = true
 
+            // A slow-loading page's evaluateJavascript callback can arrive right as the
+            // outer withTimeoutOrNull cancels this coroutine, racing finish() (JS-callback
+            // path) against invokeOnCancellation (timeout path). Both used to call
+            // webView.destroy() unconditionally -- a double destroy() crashes natively.
+            // This flag makes teardown happen exactly once no matter which path wins.
+            var finished = false
+
             fun finish(result: ExtractedArticle?) {
+                if (finished) return
+                finished = true
                 if (continuation.isActive) continuation.resume(result)
                 webView.stopLoading()
                 webView.destroy()
@@ -89,7 +100,23 @@ object ReaderExtractor {
                 }
             }
 
-            continuation.invokeOnCancellation { webView.destroy() }
+            // withTimeoutOrNull's cancellation is driven by a delayed task on
+            // kotlinx.coroutines.DefaultExecutor, NOT the calling coroutine's dispatcher --
+            // so this handler can run on a background thread despite the whole extraction
+            // being wrapped in withContext(Dispatchers.Main). Touching the WebView there
+            // crashes ("A WebView method was called on thread ..."), which is exactly what
+            // a slow/never-finishing article load (timing out after TIMEOUT_MS) hit every
+            // time -- reproduced here as the real "crash loop" root cause. Hop back to the
+            // main looper before tearing down.
+            continuation.invokeOnCancellation {
+                Handler(Looper.getMainLooper()).post {
+                    if (!finished) {
+                        finished = true
+                        webView.stopLoading()
+                        webView.destroy()
+                    }
+                }
+            }
             webView.loadUrl(url)
         }
 }
diff --git a/app/src/main/java/org/terst/feedreader/ui/ReaderScreen.kt b/app/src/main/java/org/terst/feedreader/ui/ReaderScreen.kt
index 692c198..cef17c5 100644
--- a/app/src/main/java/org/terst/feedreader/ui/ReaderScreen.kt
+++ b/app/src/main/java/org/terst/feedreader/ui/ReaderScreen.kt
@@ -231,7 +231,12 @@ private fun ReaderContent(articleId: String, onBack: () -> Unit) {
                             loadDataWithBaseURL(baseUrl, wrapHtml(content), "text/html", "UTF-8", null)
                         }
                     },
-                    update = { webView -> webView.loadDataWithBaseURL(baseUrl, wrapHtml(content), "text/html", "UTF-8", null) }
+                    update = { webView -> webView.loadDataWithBaseURL(baseUrl, wrapHtml(content), "text/html", "UTF-8", null) },
+                    // AndroidView does not destroy() the underlying WebView on its own --
+                    // without this, swiping through many articles (each page holding its own
+                    // WebView) leaks a WebView's native resources per page and eventually
+                    // exhausts memory over a long reading session.
+                    onRelease = { webView -> webView.destroy() }
                 )
             } else if (isExtracting) {
                 CircularProgressIndicator(modifier = Modifier.align(Alignment.Center))