diff --git a/build.zig b/build.zig
index f9989f1..e71c047 100644
--- a/build.zig
+++ b/build.zig
@@ -39,10 +39,17 @@ pub fn build(b: *std.Build) void {
         .optimize = optimize,
         .root_source_file = b.path("src/env.zig"),
     }) });
+    // What Enter does in the message box.
+    const composer_tests = b.addTest(.{ .root_module = b.createModule(.{
+        .target = target,
+        .optimize = optimize,
+        .root_source_file = b.path("src/composer.zig"),
+    }) });
     const test_step = b.step("test", "Run the chat's tests");
     test_step.dependOn(&b.addRunArtifact(auth_tests).step);
     test_step.dependOn(&b.addRunArtifact(tests).step);
     test_step.dependOn(&b.addRunArtifact(transcript_tests).step);
     test_step.dependOn(&b.addRunArtifact(permission_tests).step);
     test_step.dependOn(&b.addRunArtifact(env_tests).step);
+    test_step.dependOn(&b.addRunArtifact(composer_tests).step);
 }
diff --git a/src/View.zig b/src/View.zig
index 49caf9f..66d0e6a 100644
--- a/src/View.zig
+++ b/src/View.zig
@@ -18,6 +18,7 @@ const api = @import("api.zig");
 const Chat = @import("Chat.zig");
 const Commands = @import("Commands.zig");
 const Transcript = @import("Transcript.zig");
+const composer = @import("composer.zig");
 
 const entypo = dvui.entypo;
 
@@ -815,14 +816,14 @@ fn drawInput(c: *Chat) void {
         .gravity_y = 1,
         .corners = .all(14),
         .padding = .{ .x = 10, .y = 6, .w = 10, .h = 6 },
-        // A line tall to start, growing with what is typed up to six lines, then scrolling.
+        // A line tall to start, growing with what is typed up to eight lines, then scrolling.
         .min_size_content = .{ .w = 40, .h = theme.font_body.lineHeight() },
-        .max_size_content = .{ .w = std.math.floatMax(f32), .h = theme.font_body.lineHeight() * 6 },
+        .max_size_content = .{ .w = std.math.floatMax(f32), .h = theme.font_body.lineHeight() * composer.max_lines },
     });
-    // Enter sends; Shift+Enter is a new line, as the entry would make it.
+    // Enter sends on a desktop; on a phone, and with Shift, it is a new line, as the entry would make it.
     for (dvui.events()) |*e| {
         if (!te.matchEvent(e)) continue;
-        if (e.evt == .key and e.evt.key.action == .down and e.evt.key.code == .enter and !e.evt.key.mod.shift()) {
+        if (e.evt == .key and e.evt.key.action == .down and e.evt.key.code == .enter and composer.enter(e.evt.key.mod.shift(), composer.isMobile()) == .send) {
             e.handle(@src(), te.data());
             send_now = true;
         }
diff --git a/src/composer.zig b/src/composer.zig
new file mode 100644
index 0000000..22d5aec
--- /dev/null
+++ b/src/composer.zig
@@ -0,0 +1,34 @@
+//! What the message box decides, std-only so it tests without a window: what Enter does and how
+//! tall the box grows.
+const std = @import("std");
+const builtin = @import("builtin");
+
+/// Lines the message box grows to; past this it scrolls.
+pub const max_lines: f32 = 8;
+
+pub const Enter = enum { send, newline };
+
+/// Enter sends on a desktop and is a new line on a phone, where the Send button sends; Shift+Enter
+/// is a new line anywhere.
+pub fn enter(shift: bool, mobile: bool) Enter {
+    return if (shift or mobile) .newline else .send;
+}
+
+/// Whether this build runs on a phone or tablet. The SDK does not hand plugins the app's form
+/// factor yet, so this is the target's own answer: a phone's browser still reads as a desktop.
+pub fn isMobile() bool {
+    return builtin.os.tag == .ios or builtin.abi.isAndroid();
+}
+
+test "Enter sends on a desktop and breaks the line on a phone or with Shift" {
+    try std.testing.expectEqual(Enter.send, enter(false, false));
+    try std.testing.expectEqual(Enter.newline, enter(true, false));
+    try std.testing.expectEqual(Enter.newline, enter(false, true));
+    try std.testing.expectEqual(Enter.newline, enter(true, true));
+}
+
+test "this desktop build is not mobile" {
+    if (builtin.os.tag == .macos or builtin.os.tag == .linux or builtin.os.tag == .windows) {
+        if (!builtin.abi.isAndroid()) try std.testing.expect(!isMobile());
+    }
+}