Add optional window title bars for window splits

[mcrmck]

Summary

• Adds an optional, configurable title bar displayed above or below each window when multiple windows are visible in a tab
• Title bars are GPU-rendered using virtual Screen objects (same model as the tab bar) and automatically hidden when only a single window is visible
• Follows up on discussion Interest Check: Per-Window Labels/Titles Visible in Splits #9448 — implements the feature as opt-in (disabled by default) per maintainer guidance

Configuration options

# Enable/disable and position (default: none)
window_title_bar none|top|bottom

# Template (same syntax as tab_title_template)
window_title_template '{fmt.fg.red}{bell_symbol}{activity_symbol}{fmt.fg.window}{progress_percent}{title}'
active_window_title_template none

# Colors (default: none, falls back to tab bar colors)
window_title_bar_active_foreground   none
window_title_bar_active_background   none
window_title_bar_inactive_foreground none
window_title_bar_inactive_background none

# Text alignment
window_title_bar_align left|center|right

Implementation

• C layer (state.h, state.c, child-monitor.c): Each Window struct gains a window_title_render_data field with its own VAO. Cell data is uploaded to the GPU in prepare_to_render_os_window and drawn in render_prepared_os_window.
• Layout (layout/base.py): _apply_window_title_bars() shrinks each visible window's geometry by one cell height to make room for the title bar.
• Rendering (window_title_bar.py): WindowTitleBarManager manages one WindowTitleBarScreen per visible window, handling template evaluation, SGR formatting, alignment, and cleanup.
• Integration (tabs.py): Title bars update on layout changes (relayout_borders) and title changes (title_changed).
• Custom rendering: Users can place a window_title_bar.py script in their kitty config directory with a draw_window_title(data) function, exposed as {custom} in templates (same pattern as tab bar's tab_bar.py).

Changes since initial submission

Addressed all reviewer feedback from @kovidgoyal:

1. Renamed pane → window in all option names and internals
2. Use foreground/background instead of fg/bg in color option names
3. Color options use to_color_or_none, default to None, fall back to corresponding tab bar colors (active_tab_foreground, etc.) — no theme updates needed
4. Custom Python script support via window_title_bar.py in config dir (mirrors tab_bar.py pattern)
5. Default template includes bell and progress info using existing bell_on_tab and tab_activity_symbol options, matching the tab bar's default template

Test plan

• Build compiles cleanly
• All existing tests pass
• Config options parse correctly with expected defaults
• Manual testing: title bars appear/disappear correctly with splits, stack mode, layout switching, and window resizing

Ref: #9448

🤖 Generated with Claude Code

[kovidgoyal]

Before I find time to review the actual code, just some high level comments.

1. Options should not use pane but window as these are called windows not panes in kitty. Also to be consistent with the tab bar they should use foreground/background not fg/bg.

2. The color options should use to_color_or_none and when none use the corresponding colors from the tab bar. They should default to None. Otherwise all the existing kitty color themes would need to be updated with these four colors.

3. There should be a way to customise the rendering using a python script just as there is for tab bars (I dont know if this is already implemented)

4. The default template should include bell and progress information just as the default template for the tab bar does.

[mcrmck]

Thank you for your feedback.

I have pushed a commit addressing all four points:

1. Renamed all options from pane_title_* → window_title_*
2. Color options now use foreground/background instead of fg/bg
3. Color options use to_color_or_none, default to None, and fall back to the corresponding tab bar colors
4. Added custom Python script support via window_title_bar.py in the config directory (same pattern as tab_bar.py)
5. Default template now includes bell and progress info, reusing the existing bell_on_tab and tab_activity_symbol options to match the tab bar"--jq '.html_url'