Wayland: Fix erratic fallback decoration behavior

The handler for xdg_toplevel::configure treated the provided size as the
content area size when instead it is the size of the bounding rectangle
of the wl_surface and all its subsurfaces.

This caused the fallback decorations to try positioning themselves
outside themselves, causing feedback loops during interactive resizing.

Fixes #1991
Fixes #2115
Closes #2127
Related to #1914

(cherry picked from commit 0f5b095042)

CONTRIBUTORS.md

@@ -23,6 +23,7 @@ video tutorials.
  - Waris Boonyasiriwat
  - Kyle Brenneman
  - Rok Breulj
+ - TheBrokenRail
  - Kai Burjack
  - Martin Capitanio
  - Nicolas Caramelli
@@ -143,6 +144,7 @@ video tutorials.
  - Jon Morton
  - Pierre Moulon
  - Martins Mozeiko
+ - James Murphy
  - Julian Møller
  - ndogxj
  - Kristian Nielsen

README.md

@@ -162,6 +162,8 @@ information on what to include when reporting a bug.
  - [Wayland] Bugfix: A window content scale event would be emitted every time
    the window resized
  - [Wayland] Bugfix: If `glfwInit` failed it would close stdin
+ - [Wayland] Bugfix: Manual resizing with fallback decorations behaved erratically
+   (#1991,#2115,#2127)
 
 
 ## Contact

src/wl_window.c

@@ -537,10 +537,19 @@ static void xdgToplevelHandleConfigure(void* userData,
     window->wl.fullscreen = fullscreen;
 
     if (width && height)
+    {
+        if (window->wl.decorations.top.surface)
+        {
+            window->wl.pending.width  = _glfw_max(0, width - GLFW_BORDER_SIZE * 2);
+            window->wl.pending.height =
+                _glfw_max(0, height - GLFW_BORDER_SIZE - GLFW_CAPTION_HEIGHT);
+        }
+        else
         {
             window->wl.pending.width  = width;
             window->wl.pending.height = height;
         }
+    }
     else
     {
         window->wl.pending.width  = window->wl.width;