cannot control focus on a tabbed view

[thawkins]

Bug Description

My application has a tabbed view, a row of tabs enbles a set of views one a t a time.

We have a CustomTextEdit control on one view that we need to give focus when the tab is selected and the tab is mad visable. but we cannot find any way of forcing focus on our custom control when the tab is selected.

Reproducible Code (if applicable)

Environment Details

• Slint Version: latest
• Platform/OS: Fedora 43
• Programming Language: Rust
• Backend/Renderer: default

Product Impact

No response

[thawkins]

detailed description from my AI assistant

Slint Focus Management Issue - Tabbed View Focus

Issue: Slint GitHub Issue #10092

Status: Investigated and documented - waiting for Slint team response

Problem Statement

We need to automatically focus a CustomTextEdit control in a tabbed view when:

1. User clicks the tab (view becomes visible)
2. User selects view from menu
3. Tool programmatically switches to editor view

The cursor appears and works fine with mouse clicks, but keyboard input doesn't work until the user manually clicks in the editor.

Attempted Solutions (All Failed ❌)

1. Focus Trigger Property with changed Handler ❌

• Attempt: Created in-out property <int> gcode-focus-trigger in MainWindow, added changed focus-trigger => handler in CustomTextEdit, incremented trigger from menu/tab/Rust code
• Result: Changed handler never fired, property bindings weren't updating
• Root Cause: 2-way bindings (<=>) apparently prevent changed handlers from firing reliably

2. Timer with FocusScope.focus() ❌

• Attempt: Added timer in Rectangle's init callback, timer calls FocusScope.focus() after 50ms delay
• Result: Works on first view load (init runs), but fails when switching back to view
• Key Discovery: if current-view == "gcode-editor" creates Rectangle ONCE and shows/hides it. The init callback only runs once at creation, not on subsequent visibility changes

3. Using forward-focus Property ✅ (Partial Success)

• Attempt: Added forward-focus: editor-focus to GcodeEditorPanel, timer calls gcode-editor-panel.focus()
• Result: Works on first view switch! Focus properly cascades to FocusScope
• Limitation: Doesn't work when switching back because if conditional doesn't recreate element

4. Attempting One-Way Property ❌

• Attempt: Changed in-out property <int> gcode-focus-trigger to in property <int> gcode-focus-request
• Result: Can't use += on input properties, can't use 2-way binding with input properties
• Issue: Slint syntax restrictions prevent this approach

5. Trying changed Handler on Root Property ❌

• Attempt: Added changed current-view => at MainWindow level to detect view changes
• Result: Can't use changed inside conditional blocks, only at property definition level

Key Discoveries

1. If-Conditional Limitation

The if current-view == "gcode-editor" : Rectangle { } pattern shows/hides the Rectangle rather than recreating it:

• init callbacks only run once at creation
• changed handlers on properties inside don't re-fire on visibility changes
• Each time the condition toggles from false→true, the same Rectangle instance is reused

2. 2-Way Binding Behavior

Properties bound with <=> don't appear to trigger changed handlers reliably when the binding source changes

3. Layout Constraints

Absolute positioning (x, y, pos-x, pos-y) doesn't work for elements in layout containers

4. What DOES Work

• forward-focus property correctly forwards focus from parent to child FocusScope
• Timer-based focus on initial view load
• Keyboard input works perfectly once focused (even with mouse)

Current Implementation Status

Working ✅:

• Cursor visible at (1,1) on editor built with CustomTextEdit
• Keyboard input works after mouse click on control.
• forward-focus property routes focus to FocusScope correctly

Not Working ❌:

• Re-focusing when switching back to gcode-editor from another view
• changed handlers on 2-way bound properties

Workaround: Users must click once in the editor to focus it, then keyboard works normally

Recommended Solutions for Slint

1. Element Recreation in Conditionals

Make if conditionals recreate elements when the condition changes to true, not just show/hide existing elements. This would allow init callbacks to re-run and timers to restart.

2. Explicit Focus Callbacks

Add a Rust-side callback mechanism to explicitly set focus on specific UI elements by name/ID:

window.set_focus_to("element_id");

3. View/Pane Component

Create a built-in component for tabbed interfaces that handles focus management automatically

4. Focus Events

Add onVisible/onShow events that fire when conditionally-shown elements become visible:

if condition : SomeComponent {
    on-show => { /* runs each time visibility changes to true */ }
}

Environment Details

• Slint Version: Latest
• Platform: Fedora 43 (Linux)
• Language: Rust + Slint
• Project: gcodekit4 - G-code editor for CNC machines
• Related Files:
  • crates/gcodekit4-ui/ui.slint - Main UI with tabbed views
  • crates/gcodekit4-ui/ui_panels/gcode_editor.slint - CustomTextEdit implementation
  • src/main.rs - View switching logic

Questions for Slint Team

1. Is element recreation in conditionals planned for a future release?
2. Is there a recommended pattern for tabbed focus management that we may have missed?
3. Would a focus callback mechanism be considered?
4. Are there any performance implications to making conditionals recreate elements?

References

• GitHub Issue: cannot control focus on a tabbed view #10092
• Slint Focus Documentation: https://docs.slint.dev/latest/docs/slint/guide/development/focus/
• Project Repository: https://github.com/thawkins/gcodekit4

[ogoffart]

Would you perhaps have a small example code that shows the problem?