Refactor native rendering for correctness and robustness

Replaces the old viewport-based render algorithm with one that parks
the terminal at `max_offset - 1` after each pass. This lets us:

- Track scrollback correctly even when the libghostty scrollback cap
  is reached, by detecting eviction from the parked offset rather
  than scanning for specific escape sequences.
- Identify scrollback-clear (CSI 3J et al.) reliably via the
  `offset+len==total` snap-back signal — no per-byte VT scanning.
- Use libghostty dirty flags directly to identify stale lines,
  fixing a bug where promoted scrollback rows could carry outdated
  content.
- Evict old scrollback rows from the Emacs buffer in lockstep with
  libghostty when its ring buffer wraps.

OSC 8 hyperlink handling switches from storing URI strings on every
run to a lazy lookup via a `help-echo` function that calls
`ghostel--native-uri-at`. The Emacs buffer now always carries a
trailing newline so line-math is uniform across the codebase
(callers like evil-ghostel--reset-cursor-point that used
`line-number-at-pos` on `point-max` are switched to `count-lines`,
which is off-by-one-safe regardless of the trailing newline).
Cursor rendering is split out of the main render path.

Tests for `ghostel--cursor-on-empty-row-p` and
`ghostel--cursor-pending-wrap-p` (and the anchor-window clamp tests
that depended on them) are dropped — those helpers were removed
along with the old viewport algorithm. Docstrings and the
file-level render.zig comment block are refreshed to describe the
new algorithm; checkdoc imperative-form fixes for two new OSC 8
helpers; the unused `exact-point` argument to
`ghostel--anchor-window` is dropped along with a stale TODO in
fnUriAt.

Bug fixes folded in:

* fnCursorPosition / fnDebugState / fnDebugFeed: the viewport-
  restore defer sat inside `if (term.getScrollbar()) |sb| { defer
  { ... } }`, so the defer fired the moment the if-block ended —
  before the SCROLL_BOTTOM call below it. Each call left the
  viewport parked at the bottom (offset+len==total), which the
  next redraw mistook for a libghostty scrollback-clear and
  treated as a full erase + rebuild signal. With evil-ghostel
  this fired on every state transition once scrollback existed,
  causing a buffer wipe + rebuild on every i/Esc. Hoist the
  defer to function scope via a captured optional offset.

* fnUriAt: the heap-fallback `defer
  std.heap.c_allocator.free(buf)` was scoped to the inner alloc
  block, freeing the buffer before `makeString` read through the
  `heap_uri` alias. Hoist the free defer to function scope and
  add a SUCCESS check on the result so error returns from either
  `ghostty_grid_ref_hyperlink_uri` call no longer stringify
  uninitialised data. Tighten input bounds checks.

* Untabify whitespace cleanup across affected files.

Adds a regression test that anchors a marker in scrollback, calls
`ghostel--cursor-position`, redraws, and asserts the marker is
preserved — a spurious eraseBuffer collapses all markers to
point-min.

Author: emil-e

extensions/evil-ghostel/evil-ghostel.el

@@ -85,7 +85,7 @@ placement math the native module performs in `src/render.zig'."
   (when (and ghostel--term ghostel--term-rows)
     (let ((pos (ghostel--cursor-position ghostel--term)))
       (when pos
-        (let ((scrollback (max 0 (- (line-number-at-pos (point-max))
+        (let ((scrollback (max 0 (- (count-lines (point-min) (point-max))
                                     ghostel--term-rows))))
           (goto-char (point-min))
           (forward-line (+ scrollback (cdr pos)))

lisp/ghostel.el

@@ -567,8 +567,6 @@ Bump this only when the Elisp code requires a newer native module
 ;; Declare native module functions for the byte compiler
 
 (declare-function ghostel--cursor-position "ghostel-module")
-(declare-function ghostel--cursor-pending-wrap-p "ghostel-module")
-(declare-function ghostel--cursor-on-empty-row-p "ghostel-module")
 (declare-function ghostel--encode-key "ghostel-module")
 (declare-function ghostel--focus-event "ghostel-module")
 (declare-function ghostel--mode-enabled "ghostel-module")
@@ -578,11 +576,11 @@ Bump this only when the Elisp code requires a newer native module
 (declare-function ghostel--mouse-event "ghostel-module")
 (declare-function ghostel--new "ghostel-module")
 (declare-function ghostel--redraw "ghostel-module" (term &optional full))
-(declare-function ghostel--scroll-bottom "ghostel-module")
 (declare-function ghostel--set-default-colors "ghostel-module")
 (declare-function ghostel--set-palette "ghostel-module")
 (declare-function ghostel--set-size "ghostel-module")
 (declare-function ghostel--write-input "ghostel-module")
+(declare-function ghostel--native-uri-at "ghostel-module")
 
 
 ;;; Automatic download and compilation of native module
@@ -1325,7 +1323,6 @@ when `ghostel-scroll-on-input' is nil.  Call from any path where
 the user's action implies \"show me the prompt\" — typed input,
 paste, yank, drop."
   (when (and ghostel-scroll-on-input ghostel--term)
-    (ghostel--scroll-bottom ghostel--term)
     (setq ghostel--snap-requested t)
     (setq ghostel--force-next-redraw t)))
 
@@ -1570,7 +1567,6 @@ pasted using bracketed paste."
     (ghostel--flush-pending-output)
     ;; CSI H = home, CSI 2 J = erase screen, CSI 3 J = erase scrollback.
     (ghostel--write-input ghostel--term "\e[H\e[2J\e[3J")
-    (ghostel--scroll-bottom ghostel--term)
     (setq ghostel--force-next-redraw t)
     ;; Scrollback is gone; any recorded scroll position no longer
     ;; refers to real content.  Reset so the next redraw anchors
@@ -1851,6 +1847,30 @@ stripped so the copied text matches the original terminal content."
     map)
   "Keymap for clickable hyperlinks in ghostel buffers.")
 
+(defun ghostel--native-link-help-echo (window _ pos)
+  "help-echo handler for OSC8 hyperlinks. Retrieves native URI from libghostty."
+  (with-current-buffer (window-buffer window)
+    (ghostel--native-uri-at-pos pos)))
+
+(defun ghostel--native-uri-at-pos (pos)
+  "Return the native OSC8 hyperlink URI at POS."
+  (save-excursion
+    (goto-char pos)
+    (let* ((line (line-number-at-pos nil t))
+           (total (line-number-at-pos (point-max) t))
+           (row-from-bottom (- total line))
+           (col (current-column)))
+      (ghostel--native-uri-at ghostel--term row-from-bottom col))))
+
+(defun ghostel--uri-at-pos (pos)
+  "Return the URI at POS.
+If the `help-echo' property is a string, return it; otherwise fetch
+the native OSC8 URI at that position."
+  (let ((help-echo (get-text-property pos 'help-echo)))
+    (if (stringp help-echo)
+        help-echo
+      (ghostel--native-uri-at-pos pos))))
+
 (defun ghostel--open-link (url)
   "Open URL, dispatching by scheme.
 file:// URIs open in Emacs; http(s) and other schemes use `browse-url'.
@@ -1879,13 +1899,12 @@ a line suffix opens at the start of the file or directory."
 (defun ghostel-open-link-at-click (event)
   "Open the hyperlink at the mouse click EVENT position."
   (interactive "e")
-  (ghostel--open-link
-   (get-text-property (posn-point (event-start event)) 'help-echo)))
+  (ghostel--open-link (ghostel--uri-at-pos (posn-point (event-start event)))))
 
 (defun ghostel-open-link-at-point ()
   "Open the hyperlink at point."
   (interactive)
-  (ghostel--open-link (get-text-property (point) 'help-echo)))
+  (ghostel--open-link (ghostel--uri-at-pos (point))))
 
 (defun ghostel--find-next-link (from)
   "Return start position of the first hyperlink after FROM, or nil.
@@ -3156,14 +3175,7 @@ position COL columns into the first matched line."
     (when (> tr 0)
       (save-excursion
         (goto-char (point-max))
-        ;; Partial redraws can leave a trailing \n after the last row.
-        ;; Step past it so `forward-line' counts only content rows, not
-        ;; the phantom empty line that would otherwise push the viewport
-        ;; one line too deep and clip the bottom row.
-        (when (and (not (bobp))
-                   (eq (char-before) ?\n))
-          (forward-char -1))
-        (forward-line (- (1- tr)))
+        (forward-line (- tr))
         (line-beginning-position)))))
 
 (defun ghostel--active-preedit-overlay ()
@@ -3310,47 +3322,13 @@ No-op when `ghostel--snap-requested' (user input overrides)."
                    (eq (window-buffer win) buffer))
           (ghostel--reconcile-saved-position win entry))))))
 
-(defun ghostel--anchor-window (win vs pt &optional exact-point)
+(defun ghostel--anchor-window (win vs pt)
   "Pin WIN to viewport-start VS and sync its point to PT.
 Also resets pixel vscroll (pixel-scroll-precision-mode may leave a
-partial offset that would clip the top line after a redraw).
-
-Two terminal-side configurations land PT at `point-max' on the last
-visible row in a position Emacs redisplay treats as off-screen — which
-makes `scroll-conservatively' shift `window-start' up by one row,
-fighting the viewport pin and hiding the block cursor:
-
- 1. Pending-wrap: the last printed character filled the rightmost
-    column and the next print will soft-wrap (issue #138).
-
- 2. CUP park onto an empty trailing row, no pending-wrap: the TUI
-    moved the cursor via absolute positioning to a row that has no
-    written cells, so the row renders to an empty buffer line and PT
-    lands at `point-max' (issue #157).
-
-Clamp `window-point' back by one in either case so it sits inside the
-viewport; buffer-point is unaffected and subsequent redraws recapture
-the real cursor.  We must NOT clamp for a plain shell prompt where the
-cursor is legitimately at `point-max' after typing — doing so would
-draw the block cursor on the last character instead of after it
-\(issue #146).
-
-When EXACT-POINT is non-nil, set `window-point' to PT exactly.  This is
-used while a GUI input method is displaying preedit text: the candidate
-window is anchored to point, and moving it by one character is visible
-as a jump."
+partial offset that would clip the top line after a redraw)."
   (set-window-start win vs t)
   (set-window-vscroll win 0 t)
-  (set-window-point win (if (and (not exact-point)
-                                 (= pt (point-max))
-                                 (> pt (point-min))
-                                 ghostel--term
-                                 (or (ghostel--cursor-pending-wrap-p
-                                      ghostel--term)
-                                     (ghostel--cursor-on-empty-row-p
-                                      ghostel--term)))
-                            (1- pt)
-                          pt)))
+  (set-window-point win pt))
 
 (defun ghostel--restore-scrollback-window (win state)
   "Restore WIN to ws/wp recorded in STATE and push STATE to scroll-positions.
@@ -3423,8 +3401,7 @@ the candidate window does not jump while text is streaming in."
                                 (eq win preedit-window))))
                       (ghostel--anchor-window
                        win vs
-                       (if preedit-win-p preedit-point pt)
-                       preedit-win-p))
+                       (if preedit-win-p preedit-point pt)))
                   (ghostel--restore-scrollback-window
                    win (assq win non-anchored-states))))
               (when preedit-point

src/emacs.zig

@@ -224,6 +224,14 @@ pub const Env = struct {
         return self.call0(sym.@"line-end-position");
     }
 
+    pub fn lineBeginningPosition2(self: Env) Value {
+        return self.call1(sym.@"line-beginning-position", self.makeInteger(2));
+    }
+
+    pub fn pointMin(self: Env) Value {
+        return self.call0(sym.@"point-min");
+    }
+
     pub fn pointMax(self: Env) Value {
         return self.call0(sym.@"point-max");
     }
@@ -300,12 +308,15 @@ pub const Sym = struct {
     @"move-to-column": Value,
     @"erase-buffer": Value,
     @"line-end-position": Value,
+    @"line-beginning-position": Value,
+    @"point-min": Value,
     @"point-max": Value,
     @"delete-region": Value,
     @"char-before": Value,
     @"mark-marker": Value,
     @"marker-position": Value,
     @"set-marker": Value,
+    ding: Value,
 
     // Text property names
     face: Value,
@@ -330,7 +341,8 @@ pub const Sym = struct {
     @"ghostel--flush-output": Value,
     @"ghostel--set-title": Value,
     @"ghostel--debug-log-vt": Value,
-    ding: Value,
+    @"ghostel--native-uri-at": Value,
+    @"ghostel--native-link-help-echo": Value,
 };
 
 pub var sym: Sym = undefined;