From cdfa6ea2bfe1549d6a4b1d4c27e673fe07d6ce59 Mon Sep 17 00:00:00 2001
From: Ilya Fedin <fedin-ilja2010@ya.ru>
Date: Thu, 26 May 2022 17:20:53 +0400
Subject: [PATCH] Handle DPI change on Windows to avoid the window be resized
 by Qt

---
 ui/platform/win/ui_window_win.cpp | 23 ++++++++++++++++++++++-
 ui/platform/win/ui_window_win.h   |  1 +
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/ui/platform/win/ui_window_win.cpp b/ui/platform/win/ui_window_win.cpp
index eea2221..5286e27 100644
--- a/ui/platform/win/ui_window_win.cpp
+++ b/ui/platform/win/ui_window_win.cpp
@@ -9,6 +9,7 @@
 #include "ui/inactive_press.h"
 #include "ui/platform/win/ui_window_title_win.h"
 #include "ui/widgets/rp_window.h"
+#include "base/platform/win/base_windows_safe_library.h"
 #include "base/platform/base_platform_info.h"
 #include "base/integration.h"
 #include "base/debug_log.h"
@@ -36,6 +37,18 @@ constexpr auto kDWMWA_WINDOW_CORNER_PREFERENCE = DWORD(33);
 constexpr auto kDWMWA_CAPTION_COLOR = DWORD(35);
 constexpr auto kDWMWA_TEXT_COLOR = DWORD(36);
 
+UINT(__stdcall *GetDpiForWindow)(_In_ HWND hwnd);
+
+[[nodiscard]] bool GetDpiForWindowSupported() {
+	static const auto Result = [&] {
+#define LOAD_SYMBOL(lib, name) base::Platform::LoadMethod(lib, #name, name)
+		const auto user32 = base::Platform::SafeLoadLibrary(L"User32.dll");
+		return LOAD_SYMBOL(user32, GetDpiForWindow);
+#undef LOAD_SYMBOL
+	}();
+	return Result;
+}
+
 [[nodiscard]] bool IsCompositionEnabled() {
 	auto result = BOOL(FALSE);
 	const auto success = (DwmIsCompositionEnabled(&result) == S_OK);
@@ -163,7 +176,8 @@ WindowHelper::WindowHelper(not_null<RpWidget*> window)
 , _handle(ResolveWindowHandle(window))
 , _title(Ui::CreateChild<TitleWidget>(window.get()))
 , _body(Ui::CreateChild<RpWidget>(window.get()))
-, _shadow(std::in_place, window, st::windowShadowFg->c) {
+, _shadow(std::in_place, window, st::windowShadowFg->c)
+, _dpi(GetDpiForWindowSupported() ? GetDpiForWindow(_handle) : 0) {
 	Expects(_handle != nullptr);
 
 	init();
@@ -595,6 +609,13 @@ bool WindowHelper::handleNativeEvent(
 		}
 	} return true;
 
+	// should return true for Qt not to change window size
+	// when moving the window between screens
+	// change to false once runtime scale change would be supported
+	case WM_DPICHANGED: {
+		_dpi = LOWORD(wParam);
+	} return true;
+
 	}
 	return false;
 }
diff --git a/ui/platform/win/ui_window_win.h b/ui/platform/win/ui_window_win.h
index 1882a61..9ae881f 100644
--- a/ui/platform/win/ui_window_win.h
+++ b/ui/platform/win/ui_window_win.h
@@ -81,6 +81,7 @@ private:
 	rpl::event_stream<HitTestResult> _systemButtonOver;
 	rpl::event_stream<HitTestResult> _systemButtonDown;
 	std::optional<WindowShadow> _shadow;
+	rpl::variable<uint> _dpi;
 	QMargins _marginsDelta;
 	HMENU _menu = nullptr;
 	bool _updatingMargins = false;