MDI forms lost sub control box

• .NET Core Version:

• Have you experienced this same bug with .NET Framework?: No

Problem description:

• .NET 5 current master and Preview2

• .NET Core 3.1

Expected behavior:

.NET 5 variant must look like .NET Core 3.1 variant.

Minimal repro:

Run a repro app provided in #2979

[RussKie]

#4388 is a dup, I just never realised the buttons were the all along. @kirsan31 I ran your sample on a 5.0-rc2, and the issue was there, so it would be the result of bad merge of 3.1->5.0.

I do kind of feel like an archeologist, dig through an ancient codebase, not really knowing where to start. Here’s my understanding of how MDI control box is working. Posting here to provide more context, and so I can recall after I’m back from holidays.

Whenever an MDI windows is being maximised or minimised there’s a call to update MDI parent’s toolstrip: https://github.com/dotnet/winforms/blob/4bca5b30aea871a2c1761b39f8ff8128cb155119/src/System.Windows.Forms/src/System/Windows/Forms/Form.cs#L5613 If the child is being maximised - we merge its menu into the main menu of the parent (line:5671): https://github.com/dotnet/winforms/blob/4bca5b30aea871a2c1761b39f8ff8128cb155119/src/System.Windows.Forms/src/System/Windows/Forms/Form.cs#L5654-L5676 For that we create MdiControlStrip that contains three buttons: “Minimize”, “Maximize” and “Close”: https://github.com/dotnet/winforms/blob/4bca5b30aea871a2c1761b39f8ff8128cb155119/src/System.Windows.Forms/src/System/Windows/Forms/MDIControlStrip.cs#L32-L40 These buttons get native Windows images for respective operations via GetNativeMenuItemImage() call, which retrieves those via https://github.com/dotnet/winforms/blob/4bca5b30aea871a2c1761b39f8ff8128cb155119/src/System.Windows.Forms/src/System/Windows/Forms/ToolStripMenuItem.cs#L795-L800 …and fails. Because the default system menu doesn’t have any images.

Notice the menu in .NET 5.0 version doesn’t have any images:

Whilst in .NET Framework does:

Just for laughs I decided to set images manually, and voila:

#3461 addresses the issue by creating a new menu, which has images (magic?). Granted that PR isn’t complete like-for-like, and needs further work.

I wouldn’t be surprised if there are still things that I missed, and hopefully @JeremyKuhne and @weltkante can help me filling the gaps.

[RussKie]

Had two hours of “fun” bisecting, and it looks like we regressed back on 13 Nov 2019 in 07ae711 😱

I rolled back to 922e41002e66fb10d703705d9be700c8b30cd63d (my branch), the last commit on 3.1 branch before the merge, it is working as far as the broken use case is concerned:

I then rolled back to a0e12b1408ae45a3cc08dfd3e928c9ab8b5a45df (my branch), the last commit on 5.0 branch before the merge, and with global.json some trickery was able to build it. The broken use case is working correctly too:

I then build 07ae71178eda9d1e4ebf73be44879d969cea7dc5 and lo and behold the icons are there:

…3 hours bisecting later…

The images disappeared in https://github.com/dotnet/winforms/pull/2659 (21821568afe4f566f2d18cfece56cff666de1955)

There are probably multiple bugs, I was focusing on the regression.

So, yes, there are two distinct issues here:

1. the loss of icons in certain scenarios,
2. the loss of the control box in some other scenarios.