From d7472c061759aa56c43c36bdb5ce463e3ee8521d 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
---
 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 43d8706255..ec68a55389 100644
--- a/src/nvim/api/win_config.c
+++ b/src/nvim/api/win_config.c
@@ -260,9 +260,9 @@ 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 {
@@ -277,12 +277,14 @@ 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 e6d50b47f3..876b4a284c 100644
--- a/test/functional/api/window_spec.lua
+++ b/test/functional/api/window_spec.lua
@@ -2273,6 +2273,16 @@ 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)