Windows: main thread is blocked when user resizes or moves a window

[SDLBugzilla]

This bug report was migrated from our old Bugzilla tracker.

These attachments are available in the static archive:

• patch (SDL_modeless_size_move.patch, text/plain, 2014-02-06 05:21:48 +0000, 11407 bytes)
• better patch (sdlsizemove.patch, text/plain, 2014-02-20 21:05:23 +0000, 12485 bytes)

Reported in version: HG 2.1 Reported for operating system, platform: Windows (All), All

Comments on the original bug report:

On 2013-08-30 01:00:19 +0000, wrote:

When user clicks on window title or border, system generates WM_NCLBUTTONDOWN message. When DispatchMessage receives this message, it handles window resizing or moving and doesn't return until user releases mouse. It also sends WM_WINDOWPOSCHANGING to window proc. When using WinAPI directly there is no big problem that DispatchMessage blocks, because it is possible to handle WM_WINDOWPOSCHANGING message (or use WM_TIMER) to do actions that must be performed regularly. But on SDL it seems to be impossible to do anything in main thread while user moves or resizes window.

On 2014-01-20 05:04:25 +0000, Nathaniel Fries wrote:

I actually spent my weekend fixing this.

I'm not sure when I'll have time again to work on something, but I did upload my code for it, so someone should be able to whip up a patch fairly easily.

For an SDL-specific patch, I wouldn't bother with using a thread-local (SDL doesn't enable the creation of new GUI threads), sending WM_SIZING, or with MINMAXINFO at all.

It even includes (untested) code for child windows, so it should hopefully work in cases where SDL is used from a widget provided by Qt or some other toolkit.

it's this sourceforge project here: https://sourceforge.net/projects/win32loopl/

On 2014-01-20 16:48:16 +0000, Nathaniel Fries wrote:

Bug 2316 has been marked as a duplicate of this bug.

On 2014-01-20 16:54:40 +0000, Nathaniel Fries wrote:

Just a heads up, the above fix for this probably shouldn't be default behavior because it can cause resizing and moving to become choppy to the user if rendering or other main loop code takes too long. It could cause new bug reports from developers of pre-existing SDL2 applications who are simply passing on bug reports from users who updated their SDL2 dll. I'd recommend it as a feature that can be turned on or off by the programmer, and defaults to off.

On 2014-02-06 05:21:48 +0000, Nathaniel Fries wrote:

Created attachment 1549 patch

Finally found time to get around to making a proper patch. Code is mostly the same as I wrote before, but adapted for what SDL looks like internally. Doesn't make modeless behavior optional, though.

On 2014-02-09 10:08:44 +0000, Sam Lantinga wrote:

Thanks! We'll take a look at this after the 2.0.2 release. This also potentially fixes issues with dragging the titlebar when the cursor is grabbed?

On 2014-02-09 12:43:34 +0000, Nathaniel Fries wrote:

"This also potentially fixes issues with dragging the titlebar when the cursor is grabbed?"

Not sure what you mean by this. This code has to acquire mouse focus in order to receive all necessary mouse movements.

On 2014-02-09 20:36:01 +0000, Sam Lantinga wrote:

Yes, but we're in control of the movement process so we can account for our own grab state. It's not a fix, it just makes it possible to fix. :)

On 2014-02-09 23:31:57 +0000, Nathaniel Fries wrote:

I suppose that if mouse focus is lost, we should take it back. Might be a possible bug in this code. MSDN says not to call SetCapture when processing WM_CAPTURECHANGE, so I can see how this could have been a difficult issue previously. Now, of course, we can add a simple fix in SDL_PumpEvents or elsewhere, but we'd still have a chance of losing some events that way. A better fix would simply be to use the cursor pos attached to a windows message (__tagMSG::pt), compare it to the capture position, and work from there instead of WM_MOUSEMOVE. Then we won't even need to worry about mouse capture. Just a thought and haven't had a chance to test it, though.

Also, there was a (quite obvious once I noticed it) bug in my last patch. This is what I get for not testing thoroughly. When handling WM_MOUSEMOVE, I use lParam instead of the result from GetMessagePos. lParam is relative to the client area, which means it can be negative; GetMessagePos is in screen coordinates. This is what you get for not taking your time. :) here's a hand-written patch for my patch to correct this:

             if(data->in_modeless_resize)
             {
                 POINT ptPos;
+                DWORD dwPos = GetMessagePos();
-                ptPos.x = GET_X_LPARAM(lParam);
-                ptPos.y = GET_Y_LPARAM(lParam);
+                ptPos.x = GET_X_LPARAM(dwPos);
+                ptPos.y = GET_Y_LPARAM(dwPos);
                 WIN_DoResize(hwnd, data, ptPos, SDL_FALSE);
             }

On 2014-02-20 21:05:23 +0000, Nathaniel Fries wrote:

Created attachment 1568 better patch

Attached is a much better patch. I wasn't sure whether the values returned by SDL_GetWindow[Min/Max]imumSize were client size or window size, so that may need to be corrected inside WIN_DoResize.

Still doesn't add a new window flag for modeless behavior.

When mouse capture is lost, the modeless resize/movement operation is finalized. This is because: 1) Attempting to reclaim mouse capture in handling WM_CAPTURECHANGED actually crashes the program. Without mouse capture, we won't get messages for mouse movement outside the current boundaries of the window. 2) MSDN states that only the Foreground Window can capture the mouse, and presumably there's a good reason for an app to claim Foreground Window status (either in response to user input, or an application had to alert the user of something). The user will probably interact with this foreground window, even if just to remove its foreground window status, so it seems silly for SDL to continue acting as if the user is interactively resizing a Window.

On 2014-02-25 12:53:59 +0000, Sam Lantinga wrote:

Reviewing the code, it looks pretty good. I'm looking forward to trying it out after 2.0.2 is released.

The values returned by SDL_GetWindow[Min/Max]imumSize are client size.

On 2014-03-05 11:12:47 +0000, Andreas Ertelt wrote:

There's one tiny issue I experience with this code - in multi-monitor setups the window always jumps back to the primary monitor when being picked up.

Also, since this patch was submitted, handling for WM_NCLBUTTONDOWN was added - the current code would have to be added to the default-case of this patch and the return statement should be removed.

On 2014-03-06 10:06:56 +0000, Andreas Ertelt wrote:

Another minor issue is that I am receiving SDL_MOUSEMOTION events when moving or resizing the window in a way that the mouse cursor temporarily hovers the window. I also receive two of those events when just clicking and holding the window on the title or border as well as another when releasing.

On 2014-03-09 01:08:36 +0000, Nathaniel Fries wrote:

"There's one tiny issue I experience with this code - in multi-monitor setups the window always jumps back to the primary monitor when being picked up." something, isn't it? I don't know what would cause this and I don't have a multi-monitor setup to test on, so I'm afraid I'll have to leave that fix to someone else.

I've identified the likely cause of that minor issue (double SDL_MOUSEMOTION events). When WIN_DoResize is called in response to WM_MOUSEMOVE, the last argument should be SDL_FALSE instead of SDL_TRUE (SDL_TRUE indicates that it should "force" the cursor position to a correct value after resizing).

On 2014-03-11 07:11:10 +0000, Andreas Ertelt wrote:

Didn't have much time to test this, but the change you suggested caused the application to crash (quite literally).

The multi monitor issue is related to the GetSystemMetrics() call which is fed with SM_CXSCREEN/SM_CYSCREEN which limits the routine to the primary monitor. Instead you would have to use MonitorFromRect() to find the current/nearest monitor (using mouse coordinates and MONITOR_DEFAULTTONEAREST) and then retreive its size/coordinates using GetMonitorInfo(). Alternatively using SM_CXVIRTUALSCREEN/SM_CYVIRTUALSCREEN would be a quick fix, but that would make that part a bit pointless for setups where monitors don't use the same resolutions and/or aren't properly aligned.

Another sideeffect I found is that the aero features snap and shake stop working. Not quite sure how to emulate those correctly (especially since snap delivers visual feedback as well).

[HKEY_CURRENT_USER\Software\Policies\Microsoft\Windows\Explorer] "NoWindowMinimizingShortcuts" defines the state of shake.

[HKEY_CURRENT_USER\Control Panel\Desktop] "WindowArrangementActive" defines the state of snap.

On 2014-03-12 19:09:40 +0000, Nathaniel Fries wrote:

Actually MSDN makes it seem like the default maximum window tracking dimension is GetSystemMetrics(SM_C[X/Y]MAXTRACK) regardless of the monitor the window is on.

http://msdn.microsoft.com/en-us/library/windows/desktop/ms724385%28v=vs.85%29.aspx

I never knew of the shake and snap features. I've played around with them briefly, and shake appears to be relatively simple to implement using documented User32 calls (however, because I'm using documented User32 calls to change window position, they require a full redraw which appears to take longer than whatever User32 does internally - this may make the shake feature appear laggy as well as require we take some liberty with the timing). Snap would require I somehow cover the entire screen with a blue highlight, and I'm not sure how to do that without creating a window the size of the desktop (and we return to mouse focus issues).

On 2014-03-12 20:40:33 +0000, Nathaniel Fries wrote:

Actually, by playing around I think I've found a relatively simple way to highlight the entire screen, but it will require different code for windows on extra monitors so I can only guarantee something that would work on single-monitor systems. Might be awhile before I can materialize a complete fix, though.

On 2014-03-16 09:42:03 +0000, Nathaniel Fries wrote:

Believe it or not, I'm finding it harder to get shake just right than snap. I have basically functional versions of both in my little project on sourceforge now.

I won't be making another patch for SDL until I've got all the little quirks worked out though. Might be some time.

On 2019-12-07 17:00:27 +0000, Jake Del Mastro wrote:

Has there been any progress on this bug? I'm noticing this still seems to be an issue in SDL 2.0.10

On 2020-03-24 21:13:36 +0000, Ryan C. Gordon wrote:

(In reply to Jake Del Mastro from comment # 18)

Has there been any progress on this bug? I'm noticing this still seems to be an issue in SDL 2.0.10

Reading through all these comments, is this something we really want? It sounds like something that we're going to have to maintain every time Microsoft adds/changes a UI mechanic, and never get quite right, and introduce a bunch of risky behaviors, just to be more responsive when someone drags the window.

I'd be inclined to mark this WONTFIX, but I'll let Sam make that decision if he wants.

--ryan.

On 2020-04-16 16:50:42 +0000, Ron Aaron wrote:

It's not just an issue on Windows. macOS has the same problem (don't know if it's for a similar reason)

On 2020-04-16 19:19:48 +0000, Andreas Ertelt wrote:

Ryan is correct, the way this patch approaches the issue would require changes over time to stay consistent with Windows behavior and there are too many corner cases to consider.

But a problem should definitely not be marked WONTFIX just because a suggested solution is inadequate.

While I'm fairly sure there is no feasible solution that fully fixes the issue as it was reported here, the likely prime issue most people are concerned with is not being able to perform drawing operations / simulation anymore.

This could be addressed on Windows by allowing developers to register a callback (per window) to be performed on its WM_SIZING(!), WM_PAINT and likely also WM_ERASEBACKGROUND events. If this feature is used, the message loop would also have to call InvalidateRect on the window whenever no more messages are in the queue and upon completion of the callback a ValidateRect on the window would have to be issued (this is to make sure WM_PAINT events keep getting issued when nothing else is happening).

I'm confident most other platforms could be handled in a similar fashion.

This approach wouldn't affect existing programs in any way and provide developers who care about not being interrupted for an unreasonable amount of time with the means to address the issue with minimal changes and without having to hijack the window's message handler.

On 2020-04-18 13:08:58 +0000, Andreas Ertelt wrote:

I just checked my engine code and there are three more corner cases to be considered on Windows that I didn't think of anymore.

One is system/context menus, the other when a modular window is opened (eg. message box) and the last is picking the window up without moving it (can also be the case when moving isn't configured to redraw the window in Window's performance options).

I worked around all of this by starting a timer on the window that triggers the redraws. This timer is started under the following conditions:

1. When WM_SYSCOMMAND is called with the (wparam & 0xfff0) == SC_MOVE (this also happens when the the regular window menu is opened).
2. it must also be started when WM_ENTERMENULOOP is received to stop context menus from interrupting the program.
3. The WM_ENABLE message is received with a wparam of 0.

The only slight annoyance I could notice at this point is when you hold down the caption bar with the mouse, it takes a second to actually call the first timer-event. This can be slightly alleviated by allowing WM_GETICON to trigger a draw while the timer is active. The WM_GETICON-behavior has likely been introduced with Vista - I currently have no older machine to verify this on.

This redraw timer can then be deleted on the next proper WM_PAINT message received while the window is active again (WM_ENABLE).

In my program I trigger this timer at the refresh rate and make sure there is no more message like it in the queue before issuing the draw call (to avoid clogging the message queue).

I can't think of an alternative to using a timer here, being that the control over the message loop is being temporarily diverted and the only event being reliably triggered being WM_GETICOn at a 1Hz frequency. At least I couldn't find any other way to introduce events under these conditions.

On 2020-07-12 09:53:51 +0000, Jack C wrote:

Any updates to this bug? I like Andreas Ertelt's idea of introducing optional callbacks for those events. I know Blender's approach to drawing while resizing the window is handled in WM_SIZE/WM_SIZING event. There is an event dispatch call under "case WM_SIZE:" that will lead to a draw call.

You can find the code I am referring to here.

https://github.com/blender/blender/blob/404486e66c6a4ebebb085700d58b396597146add/intern/ghost/intern/GHOST_SystemWin32.cpp#L1659

[joncampbell123]

DOSBox-X developer here: The in-tree SDL1 library was modified for Win32 builds to separate the SDL window into a parent and child window, and a separate thread to manage one of them, so that moving/resizing the window or using the menus does not halt emulation. This trick is how DOSBox-X is able to continue running normally even when resizing. Feel free to adapt if desired.

https://github.com/joncampbell123/dosbox-x/tree/master/vs/sdl

[joncampbell123]

Not every SDL application needs this of course, so if it is added let it be an option on, say, SDL_Init()

[iactix]

It seems very odd that this kind of industry leading library is unable to let my code run when the window is dragged. For like a decade, from what I'm reading here? Just don't render anything, let all SDL code fail horribly, anything, but for FSM sake, don't block my code!

[playmer]

SDL doesn't know ahead of time that you're entering the message pump to be dragged for an indeterminate amount of time. This is a limitation of the design of the SDL_Event loop interacting with the Windows event loop. There are many workarounds, but they'd need to be implemented and all require changes on the part of the app.

Perhaps the design could be revisited in SDL3 to not interact poorly, but I'm not sure what it would look like.

[iactix]

I am sure the techicalities of why this is a problem are sound, and I am sure it's the usual microsoft thing that's causing it. However, the consequences are absolutely terrible. All I'm saying, all my criticism is regarding priorities. I'm sure music visualizations and things like that are loving it. If they are smart, they'll probably let the music go bwbwbwbwbw until you let the window go. Heck, I'm going mad just having my avg time measures f'ed up for seconds when I have to move the window out of the way of the console after starting all the time.

Btw. all of that needs to be combined with all I've read how you must not move the rendering or the event loop to different threads. All of this appears pretty extreme to me. Which is of course measured by the standing SDL seems to have. It's not like I would complain about some dude's engine that way. Anyway, cheerio everyone. Just felt that this whole thing needed quite the kick in the behind.

[joncampbell123]

There is a reason dragging/resizing the window or using the menus blocks execution of your SDL application.

The way Windows handles those interactions, and always has handled it going back to Windows 1.x even, is that DefWindowProc() goes into it's own event handling loop to handle that action. This of course blocks the SDL event handling loop.

The way DOSBox-X handles it is by modding the SDL library to maintain both a parent top level window and a child window inside, and then a separate thread handles message handling. If DefWindowProc() blocks for window size/move and menu interaction, then that thread is blocked while the main SDL application continues to run unimpeded.

Perhaps official SDL development can handle it differently or possibly cleaner, but that's how you can avoid the blocking issue entirely.

[iactix]

Appreciate the help, but really I'm not using a cross platform thingy that "is mainly used to handle cross platform window management" to work around window management tailored to specific platforms. The only solution that works is that SDL is just able to move a running program across the screen, even on an outlandish platform like windows.

[icculus]

We're still discussing what the appropriate way to work around this Windows limitation should be for SDL3, which is why this issue is still open.

While we discuss that, I'm going to lock this thread, as I think we have enough feedback telling us that people feel strongly about finding a resolution.

[slouken]

I've added a solution that dovetails nicely with the new main callbacks in SDL 3.0 and if you're not using that you can set an event watcher to handle expose events and draw then.

Thanks for all the feedback!

[RT2Code]

Being angry because you are completely ignorant on a topic doesn't help you or anyone else. This issue is indeed related to Win32 modal loops, and also applies if you use Win32 directly. A little search on the internet, or just reading this conversation, would have told you about that. Moreover, this problem is solved now, so I don't see the point of your intervention.

Anyway, props to the SDL team for your amazing work on this library, you don't deserve such rude comments.

[clseibold]

@RT2Code It should be the SDL team that apologizes for the rude comments themselves. If you make a multiplatform library like this, it is your responsibility to make sure that your event loop interacts correctly with ALL of the platforms. SDL had this issue for several years and they kept brushing it off, as many of the other people in this conversation have pointed out. @icculus 's comments above, berating people because they expect this library to not block on window resizes (or even holding down one of the window buttons on macOS), and even trying to compare this to someone unplugging an ethernet cable, was extremely disappointing and completely uncalled for.

And no, the problem isn't solved. It still exists in SDL2. We still have to use a workaround for SDL2. Your "example" StackOverflow link (as if StackOverflow is the best place to get programming advice, lmao) isn't using the Windows API correctly. I think you will find that the Win32 API documents what is expected and what isn't, unlike SDL. This is well explained in one of the StackOverflow answers:

When DefWindowProc handles WM_SYSCOMMAND with either SC_MOVE or SC_SIZE in the wParam, it enters a loop until the user stops it by releasing the mouse button, or pressing either enter or escape. It does this because it allows the program to render both the client area (where your widgets or game or whatever is drawn) and the borders and caption area by handling WM_PAINT and WM_NCPAINT messages (you should still receive these events in your Window Procedure).

It works fine for normal Windows apps, which do most of their processing inside of their Window Procedure as a result of receiving messages. It only effects programs which do processing outside of the Window Procedure, such as games (which are usually fullscreen and not affected anyway).

Many people have solved this problem easily. It only took the SDL team many years to fix it.

[icculus]

icculus 's comments above, berating people because they expect this library to not block

I didn't berate people, I offered several possible technical workarounds, and I locked this thread because it keeps generating unhelpful commentary like this, which is also why I'm locking it again now.

[slouken]

This is fixed for the SDL 2.30 release, in 509c70c6982b6927f5a8d4fb32f9319cbaf0c2ef