Fix four Avalonia UI regressions: mouse navigation, derived types, Enter activation, Delete

[christophwille]

Four UI regressions from the Avalonia migration (the fourth, Delete in the Analyzer pane, was reported in the #4030 thread), one commit each, each with a headless regression test that was shown red before the fix.

Fixes #4027
Fixes #4028
Fixes #4030

Mouse4 / Mouse5 no longer navigate back / forward (#4027)

WPF translated the extra mouse buttons into BrowseBack/BrowseForward commands by itself; Avalonia has no such translation and KeyBinding cannot express pointer buttons, so only Alt+Left / Alt+Right survived the migration. MainWindow now listens window-wide for PointerReleased and routes XButton1/XButton2 to the existing DockWorkspace.NavigateBackCommand/NavigateForwardCommand (handledEventsToo, since inner controls handle pointer events for their own gestures without ever using the X buttons).

Derived types not shown in tree (#4028)

DerivedTypesEntryNode.Filter reported FilterResult.Recurse, but the filter cascade's Recurse handling force-loads the node's lazy children and hides the node when all of them are hidden. A leaf derived type has no children, so every entry under "Derived Types" ended up hidden, and the hiding propagated up the whole chain. Reporting Match (what the WPF tree effectively did, since the tree ignores the search term) restores the entries and also keeps derived chains lazy instead of eagerly scanning the assembly list for every level.

Enter does nothing on a selected tree row (#4030)

Avalonia 12's default key selection triggers treat plain Enter/Space on a ListBoxItem as selection input and mark the KeyDown handled before it bubbles, so the activation handling in SharpTreeView.OnKeyDown (navigate to the member from an analyzer row, toggle a checkable row) never saw those keys. SharpTreeView now overrides ShouldTriggerSelection — the Avalonia 12 extension point for exactly this — to suppress the selection trigger precisely for the case OnKeyDown activates instead: a single selected row that is the row the key landed on. Multi-row selections keep the default collapse-to-focused-row behaviour.

Delete does nothing on a selected analyzer row (#4030, follow-up report)

The WPF SharpTreeView bound ApplicationCommands.Delete at class level, so Delete removed the top-level selection of any tree whose nodes opt in through CanDelete/Delete -- which is how a top-level analyzer entry was removed from the Analyzer pane. The Avalonia tree never received that binding; only the assembly list pane carried a hand-rolled Delete handler for assemblies. The gesture now lives in SharpTreeView.OnKeyDown again (delete the top-level selection when every node in it is deletable, then reselect the nearest survivor), which restores it for the analyzer pane and lets the pane-specific handler go.

Verification

Full ILSpy.Tests suite green locally, including the four new regression tests (Mouse_Back_And_Forward_Buttons_Navigate_The_History, Derived_Type_Entries_Stay_Visible_When_The_DerivedTypes_Node_Is_Expanded, Enter_Activates_The_Selected_Analyzer_Node, Delete_Removes_The_Selected_Top_Level_Analyzer_Node).

Written by an AI agent (Claude) on Christoph's behalf.