From b660adabce36e90cbfbb52d95715528983376ab5 Mon Sep 17 00:00:00 2001
From: Gabriel Schneider <gbrls@0x4200.cafe>
Date: Mon, 28 Sep 2026 22:02:04 -0300
Subject: A pane whose whole text fits in its rows never scrolls to keep a
 margin

The scroll margin reveals text; with none off screen there is nothing to
reveal. ensureCursorVisible and a centred look both leave such a pane at its
top. A longer file keeps helix's clamp (hxdiff and hxparity unchanged).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 src/look.zig   |  3 ++-
 src/panes.zig  | 18 +++++++++++++++++-
 src/pardes.zig | 18 ++++++++++++++++++
 3 files changed, 37 insertions(+), 2 deletions(-)

(limited to 'src')

diff --git a/src/look.zig b/src/look.zig
index b6a89b64..a413bf50 100644
--- a/src/look.zig
+++ b/src/look.zig
@@ -968,7 +968,8 @@ pub fn focusPaneLine(p: *Pardes, id: usize, at: Spot, landing: enum { center, ke
     if (pane.file) |*f| {
         if (at.line > panes.File.nlines(p.gpa, f)) return;
         if (landing == .center) {
-            const next = (at.line - 1) -| pane.rows / 2; // center, clamp at top
+            // center, clamp at top; a text that fits is shown whole
+            const next = if (pane.textFits()) 0 else (at.line - 1) -| pane.rows / 2;
             if (next != f.scroll) {
                 f.scroll = next;
                 f.syntax_dirty = true;
diff --git a/src/panes.zig b/src/panes.zig
index a7e5c668..47582692 100644
--- a/src/panes.zig
+++ b/src/panes.zig
@@ -512,6 +512,16 @@ pub const Pane = struct {
         }
     }
 
+    /// Every line of the file's text fits in the body's rows; the empty
+    /// line after a final newline is no text.
+    pub fn textFits(pane: *Pane) bool {
+        if (pane.file) |*f| {
+            const lines = File.nlines(pane.gpa, f) - @intFromBool(std.mem.endsWith(u8, f.content, "\n"));
+            return lines <= pane.rows;
+        }
+        return false;
+    }
+
     pub fn ensureCursorVisible(pane: *Pane) void {
         const off = pane.scroll();
         var in_context = false;
@@ -539,7 +549,13 @@ pub const Pane = struct {
                 pane.scrollBy(pane.body.cur_row - margin - off);
             } else if (pane.body.cur_row > last - margin) {
                 var to = pane.body.cur_row + margin;
-                if (pane.file) |*f| to = @min(to, @as(i32, @intCast(File.nlines(pane.gpa, f) -| 1)));
+                if (pane.file) |*f| {
+                    // The margin only reveals text: a pane its whole text
+                    // fits in never scrolls for it. A longer one keeps
+                    // helix's clamp at the buffer's last line.
+                    const lines: i32 = @intCast(File.nlines(pane.gpa, f));
+                    to = @min(to, if (off == 0 and pane.textFits()) @max(pane.body.cur_row, lines - 1 - @intFromBool(std.mem.endsWith(u8, f.content, "\n"))) else lines - 1);
+                }
                 if (comptime Pdf.enabled) {
                     if (pane.pdf) |pv|
                         to = @min(to, @as(i32, @intCast(modal.lineCount(pv.text) -| 1)));
diff --git a/src/pardes.zig b/src/pardes.zig
index 23341904..18eedccb 100644
--- a/src/pardes.zig
+++ b/src/pardes.zig
@@ -1354,6 +1354,24 @@ test "BootShell keep leaves the untouched boot shell a dragged document joins; r
     }
 }
 
+test "a pane its whole text fits in never scrolls to keep a margin" {
+    const p = try Pardes.init(std.testing.allocator, .{ .tty_only = true, .cols = 80, .rows = 30 });
+    defer p.deinit();
+    const pane = try p.setTestFile("one\ntwo\nthree\n");
+    // A second pane under it, as acme's placement puts one: right under the
+    // three lines, which leaves the file exactly three body rows.
+    const below = p.freeSlot().?;
+    _ = try p.openTutorView(below);
+    exec.placeNew(p, 0, 0, below, .doc);
+    p.active = 0;
+    p.sync();
+    try std.testing.expectEqual(@as(u16, 3), p.rects[0].h - pane.tag_rows);
+    for ("jj") |c| p.update(.{ .key = .{ .cp = c, .text = &.{c} } });
+    p.sync();
+    try std.testing.expectEqual(@as(i32, 2), pane.body.cur_row);
+    try std.testing.expectEqual(@as(i32, 0), pane.scroll());
+}
+
 test "dragging a column's only pane to another leaves that column in place, empty" {
     const p = try Pardes.init(std.testing.allocator, .{ .tty_only = true, .cols = 160, .rows = 32 });
     defer p.deinit();
-- 
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