fix(vfo): fix QStackedWidget cross-tab height inflation via TabStack subclass

[NF0T]

Summary

Fixes the recurring gap between the mode-selection row and filter-preset buttons visible when the Mode tab is open in DIGU or DIGL mode, particularly after switching tabs, changing bands, activating RADE, or starting with RADE as the last-used mode.

Root cause

QStackedLayout::sizeHint() returns the maximum height across all pages, not the current page's height. When the DSP tab is taller than the Mode tab — which occurs in DIGU and DIGL mode because m_digContainer (the DIGU offset control row) is visible, adding ~41 px — VfoWidget::adjustSize() over-allocates that height to m_tabStack. QStackedWidget hands the full allocation to the currently-displayed Mode tab page. The Mode tab's internal VBoxLayout distributes the surplus into filterContainer (which has QSizePolicy::Preferred and no fixed height), producing the gap between the mode buttons and the filter preset buttons.

Why the previous fix was insufficient

An earlier attempt patched showTab() with a per-page setSizePolicy(Ignored, Ignored) loop. This does not affect QStackedLayout::sizeHint() — that function calls widget->sizeHint() on each page directly, bypassing each page's size policy. The loop appeared to work because initial testing was performed in RADE (FDVU) mode, where m_digContainer is hidden (isDig is false for FDV-family modes), making both tabs equal height. The bug was not structurally present in that test. It reproduced immediately under DIGU/DIGL mode and on any resize path that bypasses showTab():

Trigger	Code location
Radio reports mode change while tab open	modeChanged lambda, line 2619
RADE activated / deactivated	setRadeActive(), line 4024
Diversity button toggled	divBtn toggled, line 1023
Radio reports diversity state change	diversityChanged signal, line 2664
ESC panel visibility change	setDiversityAllowed(), line 2061

Fix

Introduce TabStack, a QStackedWidget subclass that overrides sizeHint() and minimumSizeHint() to delegate to currentWidget() rather than the QStackedLayout maximum. The subclass is defined locally in VfoWidget.cpp; no header changes are required (m_tabStack remains QStackedWidget* and the override dispatches polymorphically).

The now-ineffective sizePolicy loop in showTab() is removed. All existing adjustSize() and resize(sizeHint()) call sites throughout the widget are correct without modification.

Test plan

• Open Mode tab in DIGU mode — confirm no gap between mode-row and filter buttons
• While Mode tab is open, switch mode to USB then back to DIGU — confirm gap does not reappear
• Open DSP tab, then open Mode tab — confirm correct height
• Activate RADE while Mode tab is open — confirm no layout shift
• Start AetherSDR with RADE as last-used mode, open Mode tab — confirm correct layout
• Perform a band change while Mode tab is open — confirm correct height after change
• Collapse and re-expand the VfoWidget, open Mode tab — confirm correct layout

🤖 Generated with Claude Code

[aethersdr-agent]

Clean, minimal fix. The TabStack subclass is the correct approach — sizeHint() and minimumSizeHint() are virtual on QWidget, so polymorphic dispatch works even though m_tabStack remains typed as QStackedWidget* in the header. The null-guard against currentWidget() is appropriate. The class is placed at file scope alongside the existing TriBtn helper, matching local convention.

Root-cause analysis in the PR body is excellent — explaining why the previous setSizePolicy(Ignored, Ignored) loop couldn't possibly work (because QStackedLayout::sizeHint() calls widget->sizeHint() directly, bypassing the page's size policy) is the kind of write-up that makes this trivially reviewable.

One minor description nit: the body says "the now-ineffective sizePolicy loop in showTab() is removed," but the diff doesn't contain that removal — showTab() on main is already clean (no sizePolicy loop). Likely just a stale sentence in the description; no code action needed.

Thanks for the thorough fix and writeup, @NF0T.