Transparent background for tooltips, dropdowns etc.

[Ciantic]

Goal is to allow create overlay windows, such as tooltips, dropdowns etc. by simply setting the backround of the window to transparent. It also allowes to do windows without decorations (#427) and would be better way for creating overlay widgets (#1577). Additionally #1249.

https://xi.zulipchat.com/#narrow/stream/147926-druid/topic/Semi-transparent.20windows.20without.20titlebars

For instance transparency is required to make tooltip shaped window:

Or dropdown window with custom shadows:

Or if you want to make your UI go full WinAmp style from 90's:

---

New API for window builder transparent:

    let main_window = WindowDesc::new(build_root_widget())
        .title("Hello World!")
        .transparent(true) // <-- NEW SETTING HERE
        .window_size((400.0, 400.0));

To get fully transparent background, hide also the title bar with show_titlebar(false).

---

Changes include a simple example in examples/transparency.rs, currently it renders a test widget like this on a transparent background:

---

Note about Windows 10 code changes: This feature is not supposed to change any behavior of non-transparent windows at all, the opaque windows still should use the same code as before.

The method I chose follows this tutorial: "Windows with C++ : High-Performance Window Layering Using the Windows Composition Engine", it's the most recent, I asked about that in window-rs repo.

Included are Mac support by @rjwittams, and Linux (GTK) support by @JAicewizard. X11 and web support is stubbed.

[JAicewizard]

you should also ad stubs for any unsupported platform

[Ciantic]

This is now ready for review 🚢, only thing I'm annoyed is the amount of commits there, I think we can't squash this as there are contributions from others, and GitHub doesn't allow squashing only some from the older commits?

I'm trying to re-base this so that commits from different people are not squashed.

[JAicewizard]

local squash + force push ;)

[Ciantic]

I'm rebasing this, currently a total mess.

[Ciantic]

Now I have rebased / squashed this.

I think it's now final. 🚢

[Ciantic]

Note for posterity, we removed two spurious println! lines with force push right there. The Github log doesn't make this very clear if we don't do real commits.

[Ciantic]

@cmyr I think this is ready for review.

[raphlinus]

This causes failure on Windows 7, STATUS_DLL_NOT_FOUND in particular. I haven't investigated in detail, but it's most likely the dependency on DirectComposition. Since it's not supported on Windows 7, it needs to be loaded as optional functions. That existed for a long while but was removed in #1191, specifically commit 290f6e4.

[Ciantic]

Okay we discussed this in Zulip, I'll change the Windows part to make the dependency to composition parts optional.

[Ciantic]

It's all green again 🟢. I don't have Windows 7 to test, and looks like the CI doesn't test that either, but when you've got time give it a test on Win 7.

If it doesn't work, I may have to get Win 7 running in vbox, I don't want to bother you to test the compilation after my changes.

[raphlinus]

It works as expected on 7 - hello works fine, and the "transparency" example isn't transparent but is otherwise functional. You might consider logging a warning in that case. So this is unblocked on me, but still requires someone to review in all the PR's glorious detail.

[Ciantic]

I added the warning if user is using Win 7 and tries transparent, and rebased, squashed my win 7 fix changes.

[cmyr]

This looks good to me, a few little observations inline but nothing blocking.

I'm curious if this has been tested on multiple monitors, at all; that seems like a possible source of complications. I'm also slightly scared of macOS, and I'll test again on my laptop with a second monitor when I get home later tonight.

Thanks in any case, this should unblock a lot of other work!

[Ciantic]

I pushed some changes.

[cmyr]

Okay, this looks good to me, thanks again!