From f2fb46ef9e3f71fa044dcdc2ca378f4b63a5d153 Mon Sep 17 00:00:00 2001
From: Sean Dewar <6256228+seandewar@users.noreply.github.com>
Date: Wed, 8 Oct 2025 17:22:29 +0100
Subject: [PATCH] fix(api): nvim_open_win default to half-size for splits
 (#36088)

Problem: after #35601, nvim_open_win incorrectly attempts to set the size of a
split window to 0 if it wasn't specified.

Solution: only attempt to set the size again if it was actually specified. This has the effect of defaulting to half the size of the parent window (or it may be equalized with other windows to make room), like before.

Fix #36080

(cherry picked from commit d7472c061759aa56c43c36bdb5ce463e3ee8521d)
---
 src/nvim/api/win_config.c           | 16 +++++++++-------
 test/functional/api/window_spec.lua | 10 ++++++++++
 2 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/src/nvim/api/win_config.c b/src/nvim/api/win_config.c
index bcad760caf..d64b9085e3 100644
--- a/src/nvim/api/win_config.c
+++ b/src/nvim/api/win_config.c
@@ -262,9 +262,9 @@ Window nvim_open_win(Buffer buffer, Boolean enter, Dict(win_config) *config, Err
       }
     }
     int flags = win_split_flags(fconfig.split, parent == NULL) | WSP_NOENTER;
+    int size = (flags & WSP_VERT) ? fconfig.width : fconfig.height;
 
     TRY_WRAP(err, {
-      int size = (flags & WSP_VERT) ? fconfig.width : fconfig.height;
       if (parent == NULL || parent == curwin) {
         wp = win_split_ins(size, flags, NULL, 0, NULL);
       } else {
@@ -279,12 +279,14 @@ Window nvim_open_win(Buffer buffer, Boolean enter, Dict(win_config) *config, Err
     });
     if (wp) {
       wp->w_config = fconfig;
-      // Without room for the requested size, window sizes may have been equalized instead.
-      // If the size differs from what was requested, try to set it again now.
-      if ((flags & WSP_VERT) && wp->w_width != fconfig.width) {
-        win_setwidth_win(fconfig.width, wp);
-      } else if (!(flags & WSP_VERT) && wp->w_height != fconfig.height) {
-        win_setheight_win(fconfig.height, wp);
+      if (size > 0) {
+        // Without room for the requested size, window sizes may have been equalized instead.
+        // If the size differs from what was requested, try to set it again now.
+        if ((flags & WSP_VERT) && wp->w_width != size) {
+          win_setwidth_win(size, wp);
+        } else if (!(flags & WSP_VERT) && wp->w_height != size) {
+          win_setheight_win(size, wp);
+        }
       }
     }
   } else {
diff --git a/test/functional/api/window_spec.lua b/test/functional/api/window_spec.lua
index c38ff75269..f1579f5345 100644
--- a/test/functional/api/window_spec.lua
+++ b/test/functional/api/window_spec.lua
@@ -1944,6 +1944,16 @@ describe('API/win', function()
       command('split')
       win = api.nvim_open_win(0, false, { win = 0, split = 'below', height = 10 })
       eq(10, api.nvim_win_get_height(win))
+
+      -- Still defaults to half-sized when no size was specified.
+      command('only')
+      eq(80, api.nvim_win_get_width(0))
+      api.nvim_open_win(0, true, { split = 'right' })
+      eq(40, api.nvim_win_get_width(0))
+
+      eq(22, api.nvim_win_get_height(0))
+      api.nvim_open_win(0, true, { split = 'below' })
+      eq(11, api.nvim_win_get_height(0))
     end)
   end)