From 2c05608a763d53306f090ab9295b2a45ecb1b964 Mon Sep 17 00:00:00 2001
From: "trop[bot]" <37223003+trop[bot]@users.noreply.github.com>
Date: Wed, 22 Apr 2026 12:33:06 +0200
Subject: [PATCH] fix: preserve transparency across setResizable toggles on
 Windows (#51217)
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After #49428 made `NativeWindowViews::CanResize()` return `resizable_`
for frameless windows (instead of `resizable_ && thick_frame_`),
`HWNDMessageHandler::SizeConstraintsChanged()` started adding
`WS_THICKFRAME` to the window style whenever `CanResize()` reported true.
`WS_THICKFRAME` is incompatible with layered (translucent) windows and
destroys their transparency.

`SetContentSizeConstraints` already guards against this by skipping
`OnSizeConstraintsChanged()` when `!thick_frame_`. `SetResizable` did
not, so toggling resizability on a transparent window (e.g.
`setResizable(false)` then `setResizable(true)`) caused the Chromium
path to add `WS_THICKFRAME` and strip transparency.

Apply the same guard in `SetResizable`. Min/max constraints are still
enforced — Chromium reads them from the widget delegate on every
`WM_GETMINMAXINFO`, independent of `SizeConstraintsChanged()`.

Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Shelley Vohr <shelley.vohr@gmail.com>
---
 shell/browser/native_window_views.cc | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/shell/browser/native_window_views.cc b/shell/browser/native_window_views.cc
index 10ed1e9844..b9ee131e4a 100644
--- a/shell/browser/native_window_views.cc
+++ b/shell/browser/native_window_views.cc
@@ -965,10 +965,18 @@ void NativeWindowViews::SetResizable(bool resizable) {
       gfx::Size window_size = GetSize();
       SetSizeConstraints(extensions::SizeConstraints(window_size, window_size));
     }
+    // Forcing OnSizeConstraintsChanged on Windows when !thick_frame_ would
+    // cause HWNDMessageHandler::SizeConstraintsChanged() to add WS_THICKFRAME
+    // based on CanResize(), which destroys transparency on layered windows.
+    // Min/max are still enforced through WM_GETMINMAXINFO directly from the
+    // widget delegate.
+#if BUILDFLAG(IS_WIN)
+    if (thick_frame_ && widget() && widget()->widget_delegate())
+      widget()->OnSizeConstraintsChanged();
+    UpdateThickFrame();
+#else
     if (widget() && widget()->widget_delegate())
       widget()->OnSizeConstraintsChanged();
-#if BUILDFLAG(IS_WIN)
-    UpdateThickFrame();
 #endif
   }
 }