(I thought I had seen this question before, but couldn't find it with a search.)
..........
Is it possible to remove [A] and markers (start and end markers) that were inserted in error or are otherwise not wanted in the timeline?
I've tried all sorts of little tricks (setting [A] at the end, etc.) but no matter what I try I end up with at least one vertical line on the timeline.
You can undo a step (up to 50 editing steps) with Ctrl+Z or reset markers with either Ctrl+Home or with alternative keyboard shortcut enabled simply by pressing R. Please mind that markers are technically always present, even if they are not shown.
(I found the other post; it referred to the older version which lacked the "Reset Markers" function.)
"Please mind that markers are technically always present, even if they are not shown."
Interesting. After reading your comment, I created a color-edited (zero saturation, gray-scale) segment with the markers in place on the timeline. Then removed the markers (Edit>Reset). The segment remained gray-scale and the eq2 in the Active box retained its "partialized" setting. Next, I deleted the eq2 from the Active box. Then re-set it, again selecting "partial." The previous markers no longer had any effect.
But could the hidden presence of the markers cause any unexpected changes to a clip?
The current position of markers is used as the default value for the start and end of a partialized filter. Once such a filter has been created, the position of markers has no effect on it.
The current position of markers is shown as labels on the buttons in the selection widget.
RE: "The current position of markers is shown as labels on the buttons in the selection widget."
Not sure if I am thinking of the same thing, but when I remove the A/B markers (Edit>Reset), and they are no longer visible, they no longer appear as values below the timeline. See attached images.
Your screenshots illustrate exactly what was written: the current position of markers is shown as labels on the buttons in the selection widget, after reset the values are 00:00:00.000 for A and 00:00:15.040 for B (the start and the end of the video). It doesn't matter whether the blue rectangle is painted or not.