The current UI in Zotero 7 beta is in a weird intermediate situation, giving the impression that highlighting with shortcuts after selecting text is working, until you click somewhere else and the highlight disappears.

Steps to reproduce:
1) Open a PDF file in the Zotero PDF viewer, without selecting any annotation tool in the annotations toolbar
2) Select some text
3) Press the keyboard shortcut Alt+1 to toggle the highlight annotation tool
4) Move the mouse: the blue select colour on the text disappears and instead the text appears highlighted in the default colour. You can also change to underline and change the annotation colour with further keyboard shortcuts (you always need to move the mouse to make the change appear).
5) Then you expect to have a way to validate the annotation that you can see, either by pressing Enter or Escape or clicking somewhere else in the PDF file. But:
- Pressing Enter does not do anything
- Pressing Escape cancels the annotation (escapes the text selection without leaving any annotation), which is probably what I would expect it to do.
- Clicking somewhere else in the PDF file does the same as pressing Escape. I would expect clicking somewhere else to validate the annotation last displayed.
- Clicking on one colour in the colour popup simply highlights with the colour you have just selected with the click

This half working current situation does not make sense. So it would probably be better to choose either:
A) Disable the annotation shortcuts when some text is highlighted
B) Allow the annotation shortcuts, but make it escape the text selection (same behaviour as clicking on the buttons at the top), so without annotating the text that was originally selected.
C) Finish properly what is already half working, giving the ability to highlight/underline the selected text with the shortcuts.

C is probably what the behaviour I would expect. The simplest option would be that pressing the shortcut Alt+1 would just highlight the text with the default colour and immediately exit the text selection with its popup, and pressing Alt+2 would just underline in the same way.

But this brings another UI confusion.
As I understand, the annotations toolbar at the top of the viewer is designed to be used only for any annotations you want to do after selecting it (future annotations). So it does not do anything on pre-existing annotations.
This is consistent with the following steps:
1) Do not select any annotation tool in the annotations toolbar
2) Select a pre-existing annotation in the PDF viewer
3) Press any of the buttons in the annotations toolbar
-> Step 3 un-selects the annotation that was selected in step 1, clearly showing that it is going to be used only for future annotations you will make.

Now, simply exchange steps 2 & 3, and you arrive in a confusing situation:
1) Press the Highlight Text button in the annotations toolbar
2) Select a pre-existing annotation in the PDF viewer
At this stage, you have the feeling that you are working on the currently selected annotation, because it shows it as selected. But at the same time, the annotation toolbar is still activated. Since both are active at the same time, I would expect that pressing buttons in the annotations toolbar would apply to the currently selected annotation. But this is not true. Clicking on any button in the annotation toolbar just un-selects the annotation, in the same way in the previous steps.

All this is a bit long, but it can probably be summarized in a simple way:
I. Either you choose to use the annotations toolbar only for future annotations.
II. Or you allow the annotations toolbar to modify ongoing annotations (text selection) and/or pre-existing annotations (when selected).
Depending on what you think is better, the UI needs to be consistent with your choice (including the shortcuts to the annotations toolbar).
I would probably prefer option II, providing that it does not bring other troubles.

A typical case where I would prefer highlighting the text selection via keyboard shortcut is when I adjust the text selection via keyboard (when that becomes available in Zotero 7). Since you are already working from the keyboard, the fastest way to highlight would just be to use a shortcut. Other shortcuts could also be used, but it feels natural to me to use the same ones as for the annotations toolbar.

[This is a follow-up discussion on this GitHub Issue and therein linked forum discussions.]