ui: harden modal widget user_data handling

Replace function-pointer-in-userdata pattern with a data context struct.

Author: kristapsk

src/ui/widget.zig

@@ -60,18 +60,28 @@ pub fn topdrop(onoff: enum { show, remove }) void {
 /// provided as btns arg to modal.
 pub const ModalButtonCallbackFn = *const fn (index: usize) void;
 
+/// Context stored in the modal window's user data.
+/// This avoids storing a function pointer directly in LVGL user_data (void*),
+/// which is not portable/safe and can break across Zig versions/ABIs.
+const ModalCtx = struct {
+    cb: ModalButtonCallbackFn,
+};
+
 /// shows a non-dismissible window using the whole screen real estate;
 /// for use in place of lv_msgbox_create.
 ///
 /// while all heap-alloc'ed resources are free'd automatically right before cb is called,
 /// the value of title, text and btns args must live at least as long as cb; they are
 /// memory-managed by the callers.
-///
-/// note: the cb callback must have @alignOf(ModalbuttonCallbackFn) alignment.
 pub fn modal(title: [*:0]const u8, text: [*:0]const u8, btns: []const [*:0]const u8, cb: ModalButtonCallbackFn) !void {
     const win = try lvgl.Window.newTop(60, title);
     errdefer win.destroy(); // also deletes all children created below
-    win.setUserdata(cb);
+
+    // Store a data pointer in LVGL user_data (safe), containing the callback.
+    const ctx = try std.heap.c_allocator.create(ModalCtx);
+    errdefer std.heap.c_allocator.destroy(ctx);
+    ctx.* = .{ .cb = cb };
+    win.setUserdata(ctx);
 
     const wincont = win.content().flex(.column, .{ .cross = .center, .track = .center });
     const msg = try lvgl.Label.new(wincont, text, .{ .pos = .center });
@@ -89,26 +99,44 @@ pub fn modal(title: [*:0]const u8, text: [*:0]const u8, btns: []const [*:0]const
         const btn = try lvgl.TextButton.new(btncont, btext);
         btn.setFlag(.event_bubble);
         btn.setFlag(.user1); // .user1 indicates actionable button in callback
-        btn.setUserdata(@ptrFromInt(i)); // button index in callback
+
+        // Store (i+1) so userdata is never null (0).
+        // In callback: btn_index = intFromPtr - 1.
+        btn.setUserdata(@ptrFromInt(i + 1));
+
         btn.setWidth(btnwidth);
         if (i == 0) {
             btn.addStyle(lvgl.nm_style_btn_red(), .{});
         }
     }
+
+    // Pass the window object pointer as event user data (edata)
     _ = btncont.on(.click, nm_modal_callback, win.lvobj);
 }
 
 export fn nm_modal_callback(e: *lvgl.LvEvent) callconv(.C) void {
-    if (e.userdata()) |edata| {
-        const target = lvgl.Container{ .lvobj = e.target() }; // type doesn't really matter
-        if (!target.hasFlag(.user1)) { // .user1 is set in modal setup
-            return;
-        }
+    const edata = e.userdata() orelse return;
 
-        const btn_index = @intFromPtr(target.userdata());
-        const win = lvgl.Window{ .lvobj = @ptrCast(edata) };
-        const cb: ModalButtonCallbackFn = @alignCast(@ptrCast(win.userdata()));
-        win.destroy();
-        cb(btn_index);
+    const target = lvgl.Container{ .lvobj = e.target() }; // type doesn't really matter
+    if (!target.hasFlag(.user1)) { // .user1 is set in modal setup
+        return;
     }
+
+    const idx_any = target.userdata() orelse return;
+    const btn_index: usize = @intFromPtr(idx_any) - 1;
+
+    const win = lvgl.Window{ .lvobj = @ptrCast(edata) };
+
+    const ctx_any = win.userdata() orelse return;
+    const ctx: *ModalCtx = @ptrCast(@alignCast(ctx_any));
+    const cb = ctx.cb;
+
+    // Destroy LVGL objects first (matches your earlier intent: free UI resources)
+    win.destroy();
+
+    // Free the context we allocated for this modal
+    std.heap.c_allocator.destroy(ctx);
+
+    // Now call user callback (may open other screens, etc.)
+    cb(btn_index);
 }