Let the page decide whether we are on the start screen

docs/INTERNALS.md

@@ -1721,6 +1721,43 @@ by `HomePage`, returned by `linkAt` in front of the address). The same page is v
 often a favourite and a recent visit both, and removing it from the list the
 pointer was not on is indistinguishable, from a sofa, from a key that did nothing.
 
+### Opening a favourite never told the app it had left the start screen
+
+His fourth report, and the best one:
+
+> when i open site that had images turned on previously from favourites and turn
+> images off it turns off images globally instead for that site … if visit website
+> by typing url turning images on/off changes setting only for that site
+
+`_atHome` was set by `ShowHome` and cleared by `Navigate` — that is, by whoever
+**asked** for a page. That misses every navigation this app does not perform
+itself, and **opening a favourite is one of them**: a tile is an ordinary link
+(which is the whole reason the start screen is a page and not a native screen), so
+the engine follows it and nothing tells the app it left. The browser then sat on
+Instagram still believing it was showing its own start screen.
+
+One wrong bit, and everything keyed to it went wrong in a different way:
+
+- the images and identity switches wrote the **browser-wide** setting instead of
+  the site's, because "no site" is exactly what the start screen is (`PageUrl`
+  returns null there, and `SiteRules.SetImages(null, …)` correctly declines);
+- key `8` looked for a tile under the pointer and found none;
+- the address bar said `start screen` over a page that was plainly not it;
+- and turning images back on flashed them for a single frame before the reload
+  put the site's own rule back — which is the symptom that proves the diagnosis,
+  because it is the global setting and the site rule disagreeing out loud.
+
+The fix is one line in the right place, and the rule behind it is the general one:
+**state that describes the page must be derived from the page, not from who asked
+for it.** `NoteShowing` is called at every load boundary and sets the flag from
+`Store.IsGenerated`, so a navigation nobody in this app initiated still moves it.
+`ShowHome` and `Navigate` still set it optimistically, since the bar should not lag
+a press, but they are no longer the authority.
+
+This had been wrong since the start screen existed. It cost nothing until #74 gave
+the app something important to decide with it, which is the usual shape: the bug
+ships years before the feature that makes it reachable.
+
 ## Settings that belong to a site, not to the browser
 
 Issues #74 and #75, from the same reporter, four days apart. Both are the same

src/elm/BrowserApp.cs

@@ -1208,6 +1208,7 @@ private void ConfigureWebView()
                 ApplyViewportFix();
                 ReportMetrics();
                 Store.RecordVisit(_web.Url, _web.Title);
+                NoteShowing(_web.Url);
 
                 // Backstop for UrlChanged, which is where a site's rules normally
                 // land. A page that somehow arrives without one still gets them,
@@ -1235,6 +1236,10 @@ private void ConfigureWebView()
                 // early enough for the images and one reload late for the identity.
                 string arrived = e.GetAsString();
                 _urlLabel.Text = Markup(arrived);
+
+                // Before the site rules, which ask whether this page is on a site
+                // at all — and "the start screen" is the answer that means no.
+                NoteShowing(arrived);
                 ApplySiteRules(arrived, true);
                 UpdateStatus();
             };
@@ -1769,6 +1774,34 @@ private void Reload(string why)
             }
         }
 
+        /// <summary>
+        /// Records what the view has actually got on it, which is the only thing
+        /// that may decide whether we are on the start screen.
+        /// </summary>
+        /// <remarks>
+        /// <c>_atHome</c> used to be set by <c>ShowHome</c> and cleared by
+        /// <c>Navigate</c> — that is, by whoever *asked* for a page, which misses
+        /// every navigation the app does not perform itself. **Opening a favourite
+        /// is one of those**: a tile is an ordinary link, so the engine follows it
+        /// and nothing tells the app it left. So the browser sat on Instagram still
+        /// believing it was showing the start screen, and issue #80's reporter found
+        /// what that costs once things started keying off it: the images and
+        /// identity switches wrote the browser-wide setting instead of the site's
+        /// (because "no site" is what the start screen is), key 8 looked for a tile
+        /// under the pointer, the address bar said "start screen", and turning
+        /// images back on flashed them for one frame before the site's own rule
+        /// reloaded them away again. Every one of those is the same wrong bit.
+        ///
+        /// A page is the start screen if it *is* the start screen. Both shapes of
+        /// that are <see cref="Store.IsGenerated"/>, and this is called at every
+        /// load boundary, so a navigation nobody in this app initiated still moves
+        /// the flag.
+        /// </remarks>
+        private void NoteShowing(string url)
+        {
+            _atHome = Store.IsGenerated(url);
+        }
+
         /// <summary>
         /// The address the page is on, or null when it is one of ours. Everything
         /// per-site keys off this, and "no site" is a real answer rather than a

src/nui/NuiBrowserApp.cs

@@ -484,7 +484,10 @@ private bool TryStartEngine()
                     // The engine reports the destination here, which is where a
                     // site's own settings go on the view — see ApplySiteRules for
                     // why that is early enough for the images and one reload late
-                    // for the identity.
+                    // for the identity. NoteShowing comes first: the rules ask
+                    // whether this page is on a site at all, and "the start screen"
+                    // is the answer that means no.
+                    NoteShowing(SafeUrl());
                     ApplySiteRules(SafeUrl(), true);
 
                     // The engine answered, so the view is not the dead kind.
@@ -521,6 +524,7 @@ private bool TryStartEngine()
                     Probe();
                     ApplyViewportFix();
                     Store.RecordVisit(SafeUrl(), SafeTitle());
+                    NoteShowing(SafeUrl());
 
                     // Backstop, for a load whose start we somehow missed. Normally
                     // a no-op: the site is already the applied one by now.
@@ -2330,6 +2334,34 @@ private void Reload(string why)
             }
         }
 
+        /// <summary>
+        /// Records what the view has actually got on it, which is the only thing
+        /// that may decide whether we are on the start screen.
+        /// </summary>
+        /// <remarks>
+        /// <c>_atHome</c> used to be set by <c>ShowHome</c> and cleared by
+        /// <c>Navigate</c> — that is, by whoever *asked* for a page, which misses
+        /// every navigation the app does not perform itself. **Opening a favourite
+        /// is one of those**: a tile is an ordinary link, so the engine follows it
+        /// and nothing tells the app it left. So the browser sat on Instagram still
+        /// believing it was showing the start screen, and issue #80's reporter found
+        /// what that costs once things started keying off it: the images and
+        /// identity switches wrote the browser-wide setting instead of the site's
+        /// (because "no site" is what the start screen is), key 8 looked for a tile
+        /// under the pointer, the address bar said "start screen", and turning
+        /// images back on flashed them for one frame before the site's own rule
+        /// reloaded them away again. Every one of those is the same wrong bit.
+        ///
+        /// A page is the start screen if it *is* the start screen. Both shapes of
+        /// that are <see cref="Store.IsGenerated"/>, and this is called at every
+        /// load boundary, so a navigation nobody in this app initiated still moves
+        /// the flag.
+        /// </remarks>
+        private void NoteShowing(string url)
+        {
+            _atHome = Store.IsGenerated(url);
+        }
+
         /// <summary>
         /// The address the page is on, or null when it is one of ours. Everything
         /// per-site keys off this, and "no site" is a real answer rather than a