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Bug 3847 - Hit Test x coordinate wrong on secondary monitor
Summary: Hit Test x coordinate wrong on secondary monitor
Status: RESOLVED FIXED
Alias: None
Product: SDL
Classification: Unclassified
Component: video (show other bugs)
Version: 2.0.5
Hardware: x86_64 Windows 10
: P2 normal
Assignee: Sam Lantinga
QA Contact: Sam Lantinga
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2017-09-25 19:35 UTC by Robert Turner
Modified: 2017-09-26 22:07 UTC (History)
1 user (show)

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Description Robert Turner 2017-09-25 19:35:37 UTC 
SDL_windowsevents.c contains code to retrieve the x and y coordinate for a requested hit test. It does this as follows:

    POINT winpoint = { (int) LOWORD(lParam), (int) HIWORD(lParam) };

LOWORD(lParam) does not correctly mask off high bits that are set if the point is on a second (or third, etc.) monitor. This effectively offsets the x-coordinate by a large value.

MSDN documentation suggests that LOWORD() and HIWORD() are the wrong macros for the task, instead suggesting we should be doing something like the following:

    POINT winpoint = { GET_X_LPARAM(lParam), GET_Y_LPARAM(lParam) };

Testing this change on my Windows 10 machine with 2 monitors gives the correct results.
Comment 1 Ozkan Sezer 2017-09-26 10:05:36 UTC 
Is this a one liner at line 956 (like below)?

--- SDL2/src/video/windows/SDL_windowsevents.c.orig
+++ SDL2/src/video/windows/SDL_windowsevents.c
@@ -953,7 +953,7 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPAR
         {
             SDL_Window *window = data->window;
             if (window->hit_test) {
-                POINT winpoint = { (int) LOWORD(lParam), (int) HIWORD(lParam) };
+                POINT winpoint = { GET_X_LPARAM(lParam), GET_Y_LPARAM(lParam) };
                 if (ScreenToClient(hwnd, &winpoint)) {
                     const SDL_Point point = { (int) winpoint.x, (int) winpoint.y };
                     const SDL_HitTestResult rc = window->hit_test(window, &point, window->hit_test_data);
Comment 2 Robert Turner 2017-09-26 10:12:33 UTC 
Yes, that one line diff matches the change I made locally. I was only able to test on Windows 10, building with Visual Studio 2017, but in my environment that was enough to fix the x-coordinate value.
Comment 3 Ozkan Sezer 2017-09-26 10:15:00 UTC 
> MSDN documentation suggests that ...

Indeed, the WM_NCHITTEST documentation supports your change
https://msdn.microsoft.com/en-us/library/windows/desktop/ms645618(v=vs.85).aspx
Comment 4 Sam Lantinga 2017-09-26 22:07:47 UTC 
Fixed, thanks!
https://hg.libsdl.org/SDL/rev/d4ac9c6d2528