diff --git a/docs/changelog.rst b/docs/changelog.rst
index 0889556a9..5e348338f 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -112,6 +112,9 @@ Detailed list of changes
 - A new :opt:`cursor_trail_color` setting to independently control the color of
   cursor trails (:pull:`8830`)
 
+- Wayland: Fix incorrect window size calculation when transitioning from
+  fullscreen to non-fullscreen with client side decorations (:iss:`8826`)
+
 0.42.2 [2025-07-16]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/glfw/wl_client_side_decorations.c b/glfw/wl_client_side_decorations.c
index a5cf896a2..05fb44e87 100644
--- a/glfw/wl_client_side_decorations.c
+++ b/glfw/wl_client_side_decorations.c
@@ -570,6 +570,11 @@ window_is_csd_capable(_GLFWwindow *window) {
     return window->decorated && !decs.serverSide && window->wl.xdg.toplevel;
 }
 
+bool
+csd_should_window_be_decorated(_GLFWwindow *window) {
+    return window_is_csd_capable(window) && window->monitor == NULL && (window->wl.current.toplevel_states & TOPLEVEL_STATE_FULLSCREEN) == 0;
+}
+
 static bool
 ensure_csd_resources(_GLFWwindow *window) {
     if (!window_is_csd_capable(window)) return false;
@@ -653,18 +658,18 @@ csd_change_title(_GLFWwindow *window) {
 
 void
 csd_set_window_geometry(_GLFWwindow *window, int32_t *width, int32_t *height) {
-    bool has_csd = window_is_csd_capable(window) && decs.titlebar.surface && !(window->wl.current.toplevel_states & TOPLEVEL_STATE_FULLSCREEN);
+    const bool include_space_for_csd = csd_should_window_be_decorated(window);
     bool size_specified_by_compositor = *width > 0 && *height > 0;
     if (!size_specified_by_compositor) {
         *width = window->wl.user_requested_content_size.width;
         *height = window->wl.user_requested_content_size.height;
         if (window->wl.xdg.top_level_bounds.width > 0) *width = MIN(*width, window->wl.xdg.top_level_bounds.width);
         if (window->wl.xdg.top_level_bounds.height > 0) *height = MIN(*height, window->wl.xdg.top_level_bounds.height);
-        if (has_csd) *height += decs.metrics.visible_titlebar_height;
+        if (include_space_for_csd) *height += decs.metrics.visible_titlebar_height;
     }
     decs.geometry.x = 0; decs.geometry.y = 0;
     decs.geometry.width = *width; decs.geometry.height = *height;
-    if (has_csd) {
+    if (include_space_for_csd) {
         decs.geometry.y = -decs.metrics.visible_titlebar_height;
         *height -= decs.metrics.visible_titlebar_height;
     }
diff --git a/glfw/wl_client_side_decorations.h b/glfw/wl_client_side_decorations.h
index 938fb53b7..915564c8c 100644
--- a/glfw/wl_client_side_decorations.h
+++ b/glfw/wl_client_side_decorations.h
@@ -13,5 +13,6 @@ void csd_free_all_resources(_GLFWwindow *window);
 bool csd_change_title(_GLFWwindow *window);
 void csd_set_window_geometry(_GLFWwindow *window, int32_t *width, int32_t *height);
 bool csd_set_titlebar_color(_GLFWwindow *window, uint32_t color, bool use_system_color);
+bool csd_should_window_be_decorated(_GLFWwindow *window);
 void csd_set_visible(_GLFWwindow *window, bool visible);
 void csd_handle_pointer_event(_GLFWwindow *window, int button, int state, struct wl_surface* surface);
diff --git a/glfw/wl_window.c b/glfw/wl_window.c
index ec9dbb032..85a6475b7 100644
--- a/glfw/wl_window.c
+++ b/glfw/wl_window.c
@@ -424,7 +424,7 @@ apply_scale_changes(_GLFWwindow *window, bool resize_framebuffer, bool update_cs
     double scale = _glfwWaylandWindowScale(window);
     if (resize_framebuffer) resizeFramebuffer(window);
     _glfwInputWindowContentScale(window, (float)scale, (float)scale);
-    if (update_csd) csd_set_visible(window, true);  // resize the csd iff the window currently has CSD
+    if (update_csd) csd_set_visible(window, csd_should_window_be_decorated(window));  // resize the csd iff the window currently has CSD
     int buffer_scale = window->wl.fractional_scale ? 1 : (int)scale;
     wl_surface_set_buffer_scale(window->wl.surface, buffer_scale);
 }
@@ -824,7 +824,7 @@ apply_xdg_configure_changes(_GLFWwindow *window) {
         int width = window->wl.pending.width, height = window->wl.pending.height;
         csd_set_window_geometry(window, &width, &height);
         bool resized = dispatchChangesAfterConfigure(window, width, height);
-        csd_set_visible(window, !(window->wl.decorations.serverSide || window->monitor || window->wl.current.toplevel_states & TOPLEVEL_STATE_FULLSCREEN));
+        csd_set_visible(window, csd_should_window_be_decorated(window));
         debug("Final window %llu content size: %dx%d resized: %d\n", window->id, width, height, resized);
     }
 
@@ -967,7 +967,7 @@ setXdgDecorations(_GLFWwindow* window)
         zxdg_toplevel_decoration_v1_set_mode(window->wl.xdg.decoration, window->decorated ? ZXDG_TOPLEVEL_DECORATION_V1_MODE_SERVER_SIDE: ZXDG_TOPLEVEL_DECORATION_V1_MODE_CLIENT_SIDE);
     } else {
         window->wl.decorations.serverSide = false;
-        csd_set_visible(window, window->decorated);
+        csd_set_visible(window, csd_should_window_be_decorated(window));
     }
 }
 
@@ -1632,7 +1632,7 @@ void _glfwPlatformSetWindowSize(_GLFWwindow* window, int width, int height)
         csd_set_window_geometry(window, &w, &h);
         window->wl.width = w; window->wl.height = h;
         resizeFramebuffer(window);
-        csd_set_visible(window, true);  // resizes the csd iff the window currently has csd
+        csd_set_visible(window, csd_should_window_be_decorated(window));  // resizes the csd iff the window currently has csd
         commit_window_surface_if_safe(window);
         inform_compositor_of_window_geometry(window, "SetWindowSize");
     }