feat(ui): add onblur to plugin markup fields

[davidc-smith]

What this changes

Adds an onblur attribute to <field> in plugin panel markup.

A plugin text field could hand its value back only on Enter (onsubmit) or on
every keystroke (onchange), so a value typed and then left by clicking
elsewhere was lost. onblur binds an Action<string> that receives the
field's current text whenever it loses keyboard focus: a click on another
control or on empty panel space, Escape, Tab, or the panel hiding or closing
while the field is focused. All of these already raised FocusLost on
UiField; markup simply never bound OnFocusLost.

• No double apply after Enter. A one-line field submits on Enter and then
  releases focus. UiField.LosingFocusToSubmit is true for that submit and its
  focus release when the text went to OnSubmit or clearonsubmit emptied the
  field, and markup's onblur skips the focus loss then. A field binding both
  callbacks never applies the same text twice and never reports the cleared
  empty text. The same holds when an onsubmit handler hides its own panel.
• onblur on its own. A field with onblur and no onsubmit still
  applies its text when Enter ends the edit.
• Unresolved names throw the same FormatException as onsubmit:
  <field onblur="{X}"> did not resolve to an Action<string> property on <Binding>.
• Additive. Markup without onblur behaves exactly as before. This is a
  plugin-facing markup addition, but AcDream.Plugin.Abstractions is unchanged
  and the headless host does not handle markup, so neither needs anything.
• Docs. docs/plugin-ui-markup.md lists onblur in the binding table and
  the field attributes, and describes when onchange, onsubmit and
  onblur fire.
• Tests. MarkupDocumentTests drives a real UiRoot (clicks, typed
  characters, keys) through all six focus-loss paths. It also covers:
  • the unresolved-name error;
  • Enter firing onsubmit once without onblur, and a later edit plus
    click-away firing onblur;
  • clearonsubmit not reporting empty text;
  • onblur without onsubmit;
  • an onsubmit handler that hides the panel.

It was verified live with a plugin panel against a local ACE server: click-away,
Escape, Tab and panel close each apply the value, and Enter applies it once.

Original behavior

This doesn't change how the client behaves in the world. It only adds a plugin
UI markup attribute.

Checklist

• dotnet build AcDream.slnx -c Release is green
• The portable test filter passes locally (see CONTRIBUTING.md). On macOS, all of it passes except 10 HostParity peer IPC tests and 2 LauncherControlTests pipe tests, which also fail on clean main (fd4c3a3). Those are test bugs, a peer hub startup race and socket paths over the macOS length limit, fixed separately in test: fix peer IPC and launcher pipe tests on macOS #205. AcDream.App.Tests passes 8594/8594.
• One topic per PR
• No code copied from ACEmulator, ACViewer, or holtburger
• No comments describing the original client's internals

https://claude.ai/code/session_014hCYK7XAeS88JWnMdEqgyb

[eriknihlen]

Thanks for adding this. I found one case we should fix before merging.

If an onblur callback hides its own panel, hiding the panel triggers focus loss again while the first callback is still running. That calls onblur repeatedly and can crash the client.

I reproduced the repeated callbacks with a guard to prevent an actual crash.

Could you make focus loss safe when the callback hides or removes its own panel, and add a regression test confirming the callback runs only once? The feature itself looks useful; this is the blocker I found.

[davidc-smith]

Thanks for catching this. Fixed in 7471831.

Cause: UiRoot.SetKeyboardFocus sent FocusLost while the field was still KeyboardFocus, and the Visible setter hides the panel before it records _visible = false. So an onblur that hid or removed its own panel reached ClearSubtreeOwnership → SetKeyboardFocus(null) → FocusLost again, recursing until the stack overflowed.

Fix:

• UiRoot.SetKeyboardFocus now clears focus before telling the old element it lost it, so a nested hide, remove or SetKeyboardFocus(null) sees focus already gone. If a FocusLost handler moves focus somewhere else, that choice stands and the original target doesn't gain focus. This covers every FocusLost handler, not just onblur.
• UiField also ignores a FocusLost that arrives while its own callback is still running, as a second guard.

Regression tests:

• FieldOnBlurThatClosesItsOwnPanelRunsOnce: onblur hides or removes its own panel, with focus leaving by a click on empty space, Escape, or the panel itself being hidden or removed. The callback runs exactly once and focus ends up null.
• FocusLostHandlerThatClearsFocusDoesNotLoseFocusAgain, FocusLostHandlerThatMovesFocusKeepsItsChoice, FieldIgnoresAFocusLostThatArrivesWhileItsCallbackRuns: the root and field guards on their own.

These tests fail on the previous commit with the callback running repeatedly. They stop re-entering after a few calls, so a regression fails the test instead of crashing the host. The branch merges cleanly into current main.

I also dropped the stray lock-file changes from a local macOS restore (07c4f69), so the PR no longer touches them.

https://claude.ai/code/session_01SY57mHJLZ6djpWZ5nP37Tw