Add control for playback marker horizontal position

[mxmilkiib]

Description

Adds a controllable parameter for adjusting the horizontal position of the playback marker/line on waveforms, allowing users to customize where the playback position indicator appears.

Motivation

Users may prefer different playback marker positions for various workflows:

• DJs mixing may prefer the marker further right to see more upcoming audio
• Radio DJs may prefer it centered or left for different visual reference
• Users with ultrawide monitors may want to adjust position for ergonomics

This addresses feature request #14288.

Changes

• Adds [Waveform],play_marker_position control (range: 0.0 = left, 1.0 = right)
• Implements ControlPotmeter in WaveformWidgetFactory for smooth value adjustment
• Exposes control in controller picker menu for MIDI/HID mapping
• Adds tooltip documentation for the control
• Preserves existing playback marker position functionality from preferences

Technical Implementation

The control is implemented as a global waveform setting that:

• Persists across sessions via configuration
• Updates all active waveform widgets when changed
• Integrates with the existing playback marker rendering system in WaveformWidgetRenderer
• Follows Mixxx control object patterns (snake_case naming for mappable controls)
• Complements the existing preferences slider without replacing it

Testing

• Code compiles successfully
• Control is accessible via MIDI/HID mapping
• UI preferences slider continues to work
• Position persists across application restarts
• All waveform widgets update when control changes

Related Issue

Fixes #14288

[daschuer]

This feature already exists. See "Play marker position" in the waveforms preferences.

[mxmilkiib]

grand, thanks, branch retired

[ronso0]

I thought this is about changing the marker pos on the fly. If yes, the pref option has no CO, or does it?

[mxmilkiib]

oh I just went by "this feature already exists". I'll reopen as there isn't a CO, I've been more focusing on a CI process the last few days than any testing, verification

[ronso0]

Waveforms are horizontal in offcial skins, though vertical waveforms are also supported

Suggested change

	tr("Adjust the horizontal position of the playback marker on the waveforms"),
	tr("Adjust the position of the playback marker on the waveforms"),

[ronso0]

same here

[ronso0]

Suggested change

	if (!m_pPlayMarkerPositionCO) {
	m_pPlayMarkerPositionCO = std::make_unique<ControlPotmeter>(
	ConfigKey(kWaveformGroup, QStringLiteral("play_marker_position")),
	0.0,
	1.0);
	m_pPlayMarkerPositionCO->set(m_playMarkerPosition);
	connect(m_pPlayMarkerPositionCO.get(),
	&ControlObject::valueChanged,
	this,
	[this](double value) {
	setPlayMarkerPosition(value);
	});
	} else {
	m_pPlayMarkerPositionCO->set(m_playMarkerPosition);
	}
	if (!m_pPlayMarkerPositionCO) {
	m_pPlayMarkerPositionCO = std::make_unique<ControlPotmeter>(
	ConfigKey(kWaveformGroup, QStringLiteral("play_marker_position")),
	0.0,
	1.0);
	m_pPlayMarkerPositionCO->set(m_playMarkerPosition);
	connect(m_pPlayMarkerPositionCO.get(),
	&ControlObject::valueChanged,
	this,
	[this](double value) {
	setPlayMarkerPosition(value);
	});
	}
	m_pPlayMarkerPositionCO->set(m_playMarkerPosition);

Why do we need a lambda here? The signal and slot signature match, no?

[ronso0]

Setting the position in preferences should also update the CO, right?

[ronso0]

move this to the cpp and use forward-declaration here