scenario: press down arrow twice. Topmost pane disappeared entirely regardless of whether it contained a drawing or not.
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5E6DJTFMGUDYGZ3WT5BGSR7KO5WWZ3LV47HNMTHWVIJLBACTRWVQC
UGEB2N5EVBSHMMJNDKYEHFYH2HHXEHJK3UWSXZB4BCVQR2N65OAAC
OTIBCAUJ3KDQJLVDN3A536DLZGNRYMGJLORZVR3WLCGXGO6UGO6AC
J3ER7DFO2TXYUMJAXZUFEHQNLFDNIXSYDTE7HEFGQ2RYB3A6RFPAC
BYG5CEMVXANDTBI2ORNVMEY6K3EBRIHZHS4QBK27VONJC5537COQC
--? print('', 'overlap', sy, sy+Margin_above+pane.height+Margin_below, 'vs', Display_settings.y, Display_settings.y+App.screen.height - Header_height)
-- assumes intervals are half-open: [lo, hi)-- https://en.wikipedia.org/wiki/Interval_(mathematics)
-- assumes intervals are half-open: [lo, hi)
-- https://en.wikipedia.org/wiki/Interval_(mathematics)
if lo1 < lo2 and hi1 > lo2 then
if lo1 <= lo2 and hi1 > lo2 then
if lo1 < hi2 and hi1 > hi2 then
if lo1 < hi2 and hi1 >= hi2 then
return lo1 > lo2 and hi1 < hi2
return lo1 >= lo2 and hi1 <= hi2