Notices become tagline bands at the top of the body

A message, a leader chord and a prompt used to share one row of body text at
the bottom of a pane, wearing the tagline font and nothing else about a
tagline. Now each one is a TagLayer of its own, emitted through the same
renderHeaderLayer the pane and column tags go through, so it gets the tagline
height, the small-font metrics, the band offset and the border for free --
none of which a body-grid row can have by changing its font role. The text is
right aligned. The prompt stays on the canonical grid because it owns a
cursor, and a cursor has to sit on a real cell.

The body starts BELOW the bands rather than under them, the way tree-sitter
context rows already worked. Pane.body_offset is how many rows they took and
Pardes.bodyTop(pane, rect) is the one place that answers "where does the body
begin" -- replacing fifteen copies of `if (tag_bottom) r.y else r.y + BOX_H`
spread across the paint, hit-test, scroll, PDF and image paths, which is what
let the bands and the text under them come adrift. Every notice is painted on
the grid as well, because the grid is what a terminal client draws and a band
it cannot see is a message it never gets; the GUI skips grid cells a tag layer
covers, so nothing is drawn twice.

Three bugs the bands exposed, fixed here:
  - a prompt band flush with the right edge put its caret one column past the
    pane, which the detached wire refuses -- so every frame was dropped for as
    long as the prompt was open. The band now reserves that column.
  - a click on a band mapped to Sel row 0, which is the TAG row: clicking
    chrome expanded a word out of the tagline and ran it as a builtin.
  - a watched file reloading under the editor changed the core without going
    through update, so needs_frame was never set and the reload was never
    drawn. Pardes.invalidate() is the name for that, and the file and theme
    reloads call it.

A session can now drive its own 9P namespace instead of being refused one:
ownMountSuffix answers what a path names inside this editors own tree and
resolve, readLimit and write serve it from memory rather than making the
syscall that never returns. The match is anchored to whole components under
the registrys 9p/pardes/<name>, because a bare /pardes/<name> anywhere in a
string would claim ~/src/pardes/<name>/README -- and, before write learned the
same trick, write the trees bytes over the real file. readFileLimit and
writeFile refuse instead, having no core to answer from.

A toggle setting SETS when given `on` or `off` and only flips when it is bare,
so the report LocationsConfig prints can be fed back as configuration and mean
what it says.

Snapshots: 97/98, from 0/98. The goldens were several commits stale and 17
scripts had stopped running; `config <line>` is a new script command that
appends to the per-script startup config, so a script that clicks body
coordinates pins `Verbose off` instead of counting the rows an announcement
moves. nested-optout is left failing on purpose: two levels of nesting prepend
vaxis F3 codepoints to typed lines, which is a real bug and is written down in
docs/divergences.md with a repro.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

1 files changed, 17 insertions, 8 deletions

diff --git a/src/gui/gui.zig b/src/gui/gui.zig
index d871e55e..c26629ab 100644
--- a/src/gui/gui.zig
+++ b/src/gui/gui.zig
@@ -1788,6 +1788,7 @@ const Gui = struct {
     scroll_pane: ?usize = null,
     scroll_rect: pardes.Rect = .{ .x = 0, .y = 0, .w = 0, .h = 0 },
     scroll_body_y: u16 = 0, // that rect's first BODY row (Tagbottom moves it)
+    scroll_body_h: u16 = 0, // and how many rows it has, notice bands taken off
     scroll_col: u16 = 0, // where the wheel turned: crossed rows are delivered
     scroll_row: u16 = 0, // THERE, not wherever the pointer has drifted to since
     scroll_delta: f32 = 0, // finite raw distance waiting for the render batch
@@ -4292,14 +4293,15 @@ fn stepScroll(g: *Gui, core: *pardes.Pardes, gpa: std.mem.Allocator) void {
     }
     g.scroll_lag = st.lag;
     g.scroll_rect = r;
-    const body_y = if (core.settings.tag_bottom) r.y else r.y + pardes.BOX_H;
+    const body_y = core.bodyTop(pane, r);
     g.scroll_body_y = body_y;
+    g.scroll_body_h = (r.h -| pardes.BOX_H) -| pane.body_offset;
     if (st.rows != 0) {
         const s = &core.surface;
         g.scroll_edge_len = 0;
-        if (r.w > config.GUTTER and r.h > pardes.BOX_H and r.x + r.w <= s.cols) {
+        if (r.w > config.GUTTER and r.h > pardes.BOX_H + pane.body_offset and r.x + r.w <= s.cols) {
             const bw = r.w - config.GUTTER;
-            const bh = r.h - pardes.BOX_H;
+            const bh = (r.h - pardes.BOX_H) - pane.body_offset;
             const k: u16 = @intCast(@min(@abs(st.rows), @as(i32, bh)));
             const srow = body_y + (if (st.rows > 0) k - 1 else bh - k);
             if (srow < s.rows) {
@@ -4327,7 +4329,11 @@ fn emitScrollRows(g: *Gui, instances: [*]CellInstance, base: u32, surface: *pard
     const x0 = r.x + config.GUTTER;
     const y0 = g.scroll_body_y;
     const bw = r.w - config.GUTTER;
-    const bh = r.h - pardes.BOX_H;
+    // The body the editor actually painted: measuring it from the rect while
+    // the origin comes from bodyTop reads past the pane by the band rows, and
+    // for the bottom-most pane past the surface, which drops the animation.
+    const bh = g.scroll_body_h;
+    if (bh == 0) return 0;
     if (x0 + bw > surface.cols or y0 + bh > surface.rows) return 0; // resized under us
     var shifted = layout;
     shifted.y_off -= g.scroll_lag * layout.h;
@@ -4359,7 +4365,7 @@ fn scrollScissor(g: *const Gui, layout: CellLayout, sw: u32, sh: u32) c.SDL_Rect
     const px = layout.x_off + @as(f32, @floatFromInt(r.x + config.GUTTER)) * layout.w;
     const py = layout.y_off + @as(f32, @floatFromInt(g.scroll_body_y)) * layout.h;
     const pw = @as(f32, @floatFromInt(r.w - config.GUTTER)) * layout.w;
-    const ph = @as(f32, @floatFromInt(r.h - pardes.BOX_H)) * layout.h;
+    const ph = @as(f32, @floatFromInt(g.scroll_body_h)) * layout.h;
     const x0 = std.math.clamp(@as(i32, @intFromFloat(@floor(px))), 0, @as(i32, @intCast(sw)));
     const y0 = std.math.clamp(@as(i32, @intFromFloat(@floor(py))), 0, @as(i32, @intCast(sh)));
     const x1 = std.math.clamp(@as(i32, @intFromFloat(@ceil(px + pw))), x0, @as(i32, @intCast(sw)));
@@ -6685,11 +6691,14 @@ fn appendPaneChrome(
     const chrome = core.chromeTheme();
     const page = overlayRgb(core.theme().bg orelse bg_default);
     for (core.panes, core.rects) |slot, r| {
-        if (slot == null or r.w <= config.GUTTER or r.h <= pardes.BOX_H) continue;
+        const pane = slot orelse continue;
+        if (r.w <= config.GUTTER or r.h <= pardes.BOX_H) continue;
         if (@as(u32, r.x) + r.w > surface.cols or @as(u32, r.y) + r.h > surface.rows) continue;
         const tag_y = if (core.settings.tag_bottom) r.y + r.h - pardes.BOX_H else r.y;
-        const body_y = if (core.settings.tag_bottom) r.y else r.y + pardes.BOX_H;
-        const body_end = body_y + r.h - pardes.BOX_H;
+        // The rail the editor painted, notice bands included -- overlaying the
+        // rows above it would tint the bands instead of the scrollbar.
+        const body_y = core.bodyTop(pane, r);
+        const body_end = body_y + ((r.h -| pardes.BOX_H) -| pane.body_offset);
         const x0 = @as(f32, @floatFromInt(r.x)) * layout.w;
         const x1 = x0 + @as(f32, @floatFromInt(r.w)) * layout.w;
         const tag_cells = surface.cells[@as(usize, tag_y) * surface.cols + r.x + pardes.TAG_TEXT_INSET ..][0 .. r.w - pardes.TAG_TEXT_INSET];