Spectrum max hold / decay 0

[srcejon]

In the docs for the spectrum decay dial, it says:

"A value of zero means no decay and thus phosphor history and max hold (red line) will be kept until the clear button (B.1.4) is pressed."

However, when I set decay to 0, max hold seems to be reset when the mouse is moved over a channel window, if the waterfall isn't being displayed.

To recreate:

• Enable max hold
• Set decay to 0
• Add a channel
• Disable waterfall (so only spectrum is displayed)
• Move mouse between spectrum and channel - max hold will be reset

[f4exb]

it happens because of this line of code https://github.com/f4exb/sdrangel/blob/master/sdrgui/gui/glspectrumview.cpp#L3306 thus the histogram data is systematically cleared here https://github.com/f4exb/sdrangel/blob/master/sdrgui/gui/glspectrumview.cpp#L3354 unless fftSizeChanged is properly set which happens only if the waterfall is displayed.

Note that this happens each time applyChanges() is called which happens when m_changesPending is set to true and happens in other occasions like spectrum window resizing ...