[Controls] Extract RectangleSelectionMarquee control with per-theme colours

[Xavier Fortin (xfortin-devolutions)]

Summary

Extracts the GroupedTileListBox drag-to-select rubber-band into a dedicated, reusable RectangleSelectionMarquee templated control, and gives each theme control over the marquee's fill and border colours.

Previously the marquee was an inline Border in the GroupedTileListBox template, tinted with SystemAccentColor via brushes defined locally in GroupedTileListBox.axaml. This meant the marquee looked identical across all themes and couldn't be restyled independently. This PR:

• Adds RectangleSelectionMarquee (TemplatedControl) with its own ControlTheme in Devolutions.AvaloniaControls, registered through DefaultControlTemplates.axaml.
• Replaces the inline Border in GroupedTileListBox with the new control (template part type updated accordingly).
• Introduces RectangleSelectionMarqueeFill / RectangleSelectionMarqueeBorderBrush resources per theme:
  • DevExpress and Linux (Yaru): accent-tinted (SystemAccentColor), matching the previous Explorer-style rubber-band.
  • macOS: neutral black/white tint (light/dark) for a more native AppKit-style selection rectangle.

Builds on the drag-to-select marquee introduced in #602.

Changes

• RectangleSelectionMarquee.axaml / .axaml.cs — new control + default template
• GroupedTileListBox.axaml / .axaml.cs — use the new control; drop local brush definitions
• DefaultControlTemplates.axaml — register the new control theme
• ThemeResources.axaml (DevExpress, Linux, macOS) — per-theme marquee brushes

[Copilot]

Pull request overview

Extracts the drag-selection marquee into a reusable themed control.

Changes:

• Adds RectangleSelectionMarquee and its default template.
• Adds DevExpress, Linux, and macOS theme brushes.
• Updates GroupedTileListBox to use the new control.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
MacOS ThemeResources.axaml	Adds light/dark neutral brushes.
Linux ThemeResources.axaml	Adds accent-based brushes.
DevExpress ThemeResources.axaml	Adds accent-based brushes.
DefaultControlTemplates.axaml	Registers the control theme.
RectangleSelectionMarquee.axaml.cs	Defines the control type.
RectangleSelectionMarquee.axaml	Defines its visual template.
GroupedTileListBox.axaml.cs	Updates the template-part type.
GroupedTileListBox.axaml	Replaces the inline border.

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[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

src/Devolutions.AvaloniaControls/Controls/RectangleSelectionMarquee.axaml:9

• These two keys are only defined by the custom theme packages, but DefaultControlTemplates.axaml is also loaded directly by SampleApp's Fluent and Simple themes (samples/SampleApp/App.axaml:33-45). In those supported configurations both dynamic lookups are unresolved, so the marquee has neither fill nor border and drag selection becomes visually invisible. Please provide fallback brushes in Devolutions.AvaloniaControls while allowing each custom theme to override them.

    <Setter Property="Background" Value="{DynamicResource RectangleSelectionMarqueeFill}" />
    <Setter Property="BorderBrush" Value="{DynamicResource RectangleSelectionMarqueeBorderBrush}" />

[randy-but-a-ro]

🤖 Pull request was approved automatically: the AI review is complete and all its review threads are resolved. 🎉

Integration Details

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