Have Text.pane return nothing for a header instead of trapping

A column's or the workspace's tag has no pane, and asking one for it hit
unreachable; every caller guarded against that by checking `what` first.
An optional makes each caller say what it does without a pane, so a
forgotten guard no longer compiles into a crash.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 17 insertions, 13 deletions

diff --git a/src/Text.zig b/src/Text.zig
index d4b6d048..61f238ae 100644
--- a/src/Text.zig
+++ b/src/Text.zig
@@ -76,13 +76,14 @@ ed_redo: [Terminal.history_max]Terminal.Snapshot = undefined,
 ed_redo_len: usize = 0,
 
 /// The window this text belongs to (acme's t->w). A column's or the
-/// workspace's tag belongs to no pane; nothing asks it for one.
-pub fn pane(t: *Text) *Pane {
+/// workspace's tag belongs to none, and the optional makes every caller
+/// say what it does then.
+pub fn pane(t: *Text) ?*Pane {
     return switch (t.what) {
         .body => @alignCast(@fieldParentPtr("body", t)),
         .tag => @alignCast(@fieldParentPtr("tag", t)),
         .input => @alignCast(@fieldParentPtr("input", t)),
-        .column, .workspace => unreachable,
+        .column, .workspace => null,
     };
 }
 
@@ -98,7 +99,7 @@ pub fn deinit(t: *Text, gpa: std.mem.Allocator) void {
 /// the tagline follows its caret where it is drawn, and a prompt's answer is
 /// drawn whole.
 pub fn show(t: *Text) void {
-    if (t.what == .body) t.pane().ensureCursorVisible();
+    if (t.what == .body) if (t.pane()) |w| w.ensureCursorVisible();
 }
 
 pub fn toModalCursor(t: *Text) modal.Cursor {
@@ -526,8 +527,9 @@ pub fn visualMove(
 pub fn halfPageMove(t: *Text, text: []const u8, range: modal.Selection, down: bool) void {
     // a view to move through is a body's; a tag or an answer is all shown
     if (t.what != .body) return;
-    const half: i32 = @max(1, @divTrunc(@as(i32, t.pane().rows), 2));
-    t.pane().scrollBy(if (down) half else -half);
+    const w = t.pane() orelse return;
+    const half: i32 = @max(1, @divTrunc(@as(i32, w.rows), 2));
+    w.scrollBy(if (down) half else -half);
     verticalMove(t, text, range, down, @intCast(half));
 }
 
@@ -536,9 +538,10 @@ pub fn halfPageMove(t: *Text, text: []const u8, range: modal.Selection, down: bo
 pub fn scrollViewMove(t: *Text, text: []const u8, range: modal.Selection, delta: i32) void {
     // a view to move through is a body's; a tag or an answer is all shown
     if (t.what != .body) return;
-    const margin: i32 = @min(config.scroll_off, @divTrunc(@as(i32, t.pane().rows) - 1, 2));
-    t.pane().scrollBy(delta);
-    const top = t.pane().scroll();
+    const w = t.pane() orelse return;
+    const margin: i32 = @min(config.scroll_off, @divTrunc(@as(i32, w.rows) - 1, 2));
+    w.scrollBy(delta);
+    const top = w.scroll();
     const last_row: i32 = @intCast(File.textLineCount(t, text) - 1);
     const cur = modal.selectionCursor(text, range);
     if (delta > 0) {
@@ -548,7 +551,7 @@ pub fn scrollViewMove(t: *Text, text: []const u8, range: modal.Selection, delta:
         const anchor = if (t.select) range.anchor else head;
         setRange(t, text, 0, .{ .anchor = anchor, .head = head }, false);
     } else {
-        const snap: i32 = @max(0, @min(top + @as(i32, t.pane().rows) - margin - 1, last_row));
+        const snap: i32 = @max(0, @min(top + @as(i32, w.rows) - margin - 1, last_row));
         const head = File.textLineStart(t, text, @intCast(snap));
         if (head >= cur) return;
         const anchor = if (t.select) range.anchor else head;
@@ -560,10 +563,11 @@ pub fn scrollViewMove(t: *Text, text: []const u8, range: modal.Selection, delta:
 pub fn gotoWindow(t: *Text, text: []const u8, range: modal.Selection, which: enum { top, center, bottom }, cnt: usize) void {
     // a view to move through is a body's; a tag or an answer is all shown
     if (t.what != .body) return;
-    const margin: i32 = @min(config.scroll_off, @divTrunc(@as(i32, t.pane().rows) - 1, 2));
-    const top = t.pane().scroll();
+    const w = t.pane() orelse return;
+    const margin: i32 = @min(config.scroll_off, @divTrunc(@as(i32, w.rows) - 1, 2));
+    const top = w.scroll();
     const last_row: i32 = @intCast(File.textLineCount(t, text) - 1);
-    const last_vis: i32 = @min(@as(i32, t.pane().rows) - 1, last_row - top);
+    const last_vis: i32 = @min(@as(i32, w.rows) - 1, last_row - top);
     const n: i32 = @intCast(cnt - 1);
     var vline: i32 = switch (which) {
         .top => top + margin + n,