fix(dock): tabbed panel groups render a blank gap above their content

[MartenH]

Problem

A dock panel group with 2+ tabs (e.g. created by dragging one panel onto another via the center/tabify drop zone) renders a large empty band between the tab bar and the panel content. Single-tab groups are fine, so it only appears once a group has more than one tab.

Cause

In view_dock_layout.v, the per-tab separator is built as:

tab_buttons << column(width: 1, sizing: fixed_fill, color: color_sep)

That's a height-fill child placed inside the fit-height dock_tab_bar row (sizing: fill_fit). A fill child in a fit container makes the row expand to fill the available group height instead of fitting the tabs, so the tab bar balloons and pushes the content down. The separator is only added between tabs, which is why the gap appears only with 2+ tabs.

Fix

Give the separator a definite height (fixed_fixed) instead of fixed_fill, so the tab bar fits its tabs again. The divider is still drawn.

tab_buttons << column(width: 1, height: 20, sizing: fixed_fixed, color: color_sep)

Repro

Put two panels in one group:

dock_panel_group('g', ['a', 'b'], 'a')

Before: a blank band sits above the content. After: content sits directly under the tab bar.

Verified in a real ~25-panel dockable app: tabbing any two panels together now renders cleanly.

[GGRei]

@MartenH You need to rebase/sync your branch with the current vlang/gui main branch, because your PR is still using the old Windows CI workflow with V_VERSION: 0.5.1 Thanks!

[MartenH]

@GGRei Done