docs: record the three review findings as issues and memory

#15 every tab showed the first file opened — upstream does not diff text
on update, and the coordinator that works around it looks redundant
enough that the next person would delete it without reading why.
MEMORY.md #18 exists for that, with the upstream code quoted and the
.id(document.id) fallback recorded.

#16 a selected folder's chevron did nothing — the direct consequence of
driving expansion off the selection change. Cross-linked from #12, which
explains why the tap gesture was removed in the first place.

#17 the gutter drew through the translucent tab strip. Marked honestly as
a hypothesis rather than a diagnosis: it cannot be reproduced
synthetically, and the competing explanation is the editor's content
insets, with the next thing to try written down.

CURRENT_STATE now records that the visual review happened and found
three things, none of which were visible without someone looking at the
running app. ARCHITECTURE accounts for the fourth upstream-touching file.

Author: yan

docs/ARCHITECTURE.md

@@ -181,6 +181,10 @@ Anything that grows a filesystem dependency should use it.
 
 - `SourceEditor` and `EditorTheme.Attribute` — contained in `CodeEditorView.swift` and
   `EditorTheme+Bridge.swift`. Both are upstream, both are pre-1.0. See [STACK.md](STACK.md).
+- `DocumentTextCoordinator` — a `TextViewCoordinator` that captures the live
+  `TextViewController` so `CodeEditorView` can reload its text. Exists because
+  `updateNSViewController` does not diff the text behind the binding. A fourth upstream-type
+  file is expected and accounted for; see [MEMORY.md #18](MEMORY.md).
 - `SourceEditorState.findPanelVisible` — `EditorState.isFindVisible` is our flag,
   `CodeEditorView` syncs it both ways, and the upstream coordinator opens the panel when
   state disagrees with it. The panel stays the authority on its own visibility.

docs/CURRENT_STATE.md

@@ -43,7 +43,14 @@ Create file, create folder, rename, delete, reveal in Finder, copy path. Dotfile
 New items land beside the active document, or at the workspace root.
 
 **Editing.** `SourceEditor` from CodeEditSourceEditor 0.15, bound two-way to
-`Document.content`. Tree-sitter highlighting across roughly 25 languages, line-number
+`Document.content`. Three defects found in visual review of [#14](https://github.com/Yan123-tech/code-editor/issues/14)
+were fixed: every tab showed the first file opened
+([#15](https://github.com/Yan123-tech/code-editor/issues/15) — upstream does not diff text on
+update), a selected folder's chevron did nothing
+([#16](https://github.com/Yan123-tech/code-editor/issues/16)), and the gutter drew through the
+translucent tab strip while rubber-band scrolling
+([#17](https://github.com/Yan123-tech/code-editor/issues/17) — fix applied, not yet
+confirmed). Tree-sitter highlighting across roughly 25 languages, line-number
 gutter, bracket-pair emphasis, indentation and reformat-at-column, word wrap, undo stack
 shared with the document model. Peripherals are configured explicitly: gutter on, minimap
 off by default with a toggle, folding ribbon and invisibles toggleable, smart quotes and
@@ -189,8 +196,10 @@ feel urgent, but it is C interop behind a protocol and nothing else depends on i
 
 ### Housekeeping before the next branch
 
-- [#14](https://github.com/Yan123-tech/code-editor/issues/14) has one unchecked box: a human
-  visual review of the redesign. Everything else about it is verified except that.
+- [#16](https://github.com/Yan123-tech/code-editor/issues/16) and
+  [#17](https://github.com/Yan123-tech/code-editor/issues/17) are fixed but were not driven in
+  the running app — synthetic accessibility events do not reach SwiftUI rows, and the gutter
+  overlap only appears mid-gesture. Both need a second look.
 - `Scripts/build-app.sh` needs running from a clean checkout before it is believed again.
   See [MEMORY.md #16](MEMORY.md#16-scriptsbuild-appsh-only-fails-from-a-clean-checkout).
 

docs/MEMORY.md

@@ -320,6 +320,37 @@ in the UI.
 
 ---
 
+## 18. `SourceEditor` never diffs its text on update
+
+`SourceEditor.updateNSViewController` compares language, configuration and highlight
+providers. It does **not** compare the text behind its `Binding<String>`. Text is pushed once,
+in `makeNSViewController`:
+
+```swift
+case .binding(let binding):
+    controller.textView.setText(binding.wrappedValue)
+```
+
+SwiftUI reuses the view controller, so `make` is not called again when the document changes.
+Every tab therefore shows the first file opened, until you switch to a file of a *different
+language*, which forces the reload.
+
+**`DocumentTextCoordinator` exists for this and only this.** It captures the controller in
+`prepareCoordinator` — the one hook that hands it out — and `CodeEditorView` calls
+`reload(text:)` on `document.id` change. It compares against `textView.string` first, so
+reloading identical text is free, and `Document.setContent` guards on equality so the
+write-back records no edit.
+
+Do not remove it because it looks redundant — it is not, and the symptom is severe enough to
+look like a broken app. If upstream 0.16 diffs text on update, delete the coordinator then.
+
+`.id(document.id)` on the editor is the fallback if this ever stops working: correct, but it
+rebuilds the tree-sitter highlighter on every tab switch and discards scroll and cursor state.
+
+---
+
+---
+
 ## Quick reference
 
 | Question | Answer |
@@ -336,4 +367,5 @@ in the UI.
 | Can I derive isDark from colours? | No. #11. |
 | Why did the status bar say "1 line selected"? | #17. A caret is not a selection. |
 | Does `Scripts/build-app.sh` build? | Only since the fix in #16. Run it from clean to check. |
+| Why do all my tabs show the same file? | #18. `SourceEditor` does not diff text on update. |
 | Where do I write "this does not work"? | Here, and [CURRENT_STATE.md](CURRENT_STATE.md). |