Fix clipped progress ring in tab when tab title is too long

[jonathan-meier]

It turns out that the negative margin for the progress ring is causing
the clipping in case the tab title gets too long:

terminal/src/cascadia/TerminalApp/TabHeaderControl.xaml

Lines 18 to 27 in 43dbbd5

	<mux:ProgressRing x:Name="HeaderProgressRing"
	Width="15"
	Height="15"
	MinWidth="0"
	MinHeight="0"
	Margin="-7.5,0,8,0"
	IsActive="{x:Bind TabStatus.IsProgressRingActive, Mode=OneWay}"
	IsIndeterminate="{x:Bind TabStatus.IsProgressRingIndeterminate, Mode=OneWay}"
	Visibility="{x:Bind TabStatus.IsProgressRingActive, Mode=OneWay}"
	Value="{x:Bind TabStatus.ProgressValue, Mode=OneWay}" />

The negative margin was introduced in #8113 because the progress ring is
supposed to replace the tab icon but the TabView still reserves space
even if no icon is set (see
#8133 (comment)).
However, it is not actually the TabView reserving space even when
there is no icon, but a workaround for a crash in the
IconPathConverter that returns a BitmapIconSource with a nullptr
source instead of a nullptr IconSource:

terminal/src/cascadia/TerminalSettingsModel/IconPathConverter.cpp

Lines 143 to 154 in 43dbbd5

	if (!iconSource)
	{
	// Set the default IconSource to a BitmapIconSource with a null source
	// (instead of just nullptr) because there's a really weird crash when swapping
	// data bound IconSourceElements in a ListViewTemplate (i.e. CommandPalette).
	// Swapping between nullptr IconSources and non-null IconSources causes a crash
	// to occur, but swapping between IconSources with a null source and non-null IconSources
	// work perfectly fine :shrug:.
	BitmapIconSource<TIconSource>::type icon;
	icon.UriSource(nullptr);
	iconSource = icon;
	}

The workaround in IconPathConverter could probably be removed as I did
not find any instance where it is still used in a way that could trigger
the mentioned crash, but I did not dare to just remove it as I do not
know enough about the code by far. Hence, I opted to just locally
instantiate the IconSource with a nullptr directly in TerminalTab.

Fixes #8910

[jonathan-meier]

@microsoft-github-policy-service agree

[DHowett]

Thanks for doing this! It seems so straightforward when you lay it out like that...

[carlos-zamora]

Love it! Thanks!

[carlos-zamora]

@msftbot merge this in 5 minutes

[ghost]

Hello @carlos-zamora!

Because you've given me some instructions on how to help merge this pull request, I'll be modifying my merge approach. Here's how I understand your requirements for merging this pull request:

• I won't merge this pull request until after the UTC date Mon, 10 Oct 2022 17:41:24 GMT, which is in 5 minutes

If this doesn't seem right to you, you can tell me to cancel these instructions and use the auto-merge policy that has been configured for this repository. Try telling me "forget everything I just told you".

[ghost]

🎉Windows Terminal Preview v1.16.3463.0 and v1.16.3464.0 has been released which incorporates this pull request.:tada:

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