The latest Firefox version (138.0.1 for me) introduced the new feature to open the collapsed vertical tabs when the mouse hover over the tabs. However, this only switches between the collapsed and expanded view, which in itself would not sound problematic, but when all the sidebar tools enabled, the tools are arranged vertically when closed, and horizontally when opened, even when hovering over the tabs with the mouse, causing the tab that the user hovers over jump out of the cursor, leading to a disorienting user experience. The jump worsens when there are pinned tabs too. The following image demonstrates these two stages (showing only the tools, that is the bottom of the sidebar):
What I suggest, is to make these tabs/tools use up as much vertical space when collapsed when they would use up when expanded, only when expanding by hovering with the mouse cursor is enabled, as to preserve the current behavior when the mouse hovering feature is turned off. So for instance, when in collapsed, only the sidebar configuration gear is visible, but when expanded, all the tools appear next to each other, in a single line. But if there are so much tools, that the expanded state would take up two rows of buttons (I think this is more applicable to pinned tabs), then the one more tool would appear above the gear, that otherwise would be the first in that row. This could be extrapolated to 3 and more rows. This would make it so, when the user hovers over a tab, pinned tab or tool, the expansion of the tab bar would not push a different tab, pinned tab or tool under the cursor, making the user accidentally click on a different tab, pinned tab or tool, frustrating them in the process. And when the pinned tab or tool isn't in view, they would hover over the tab bar, and then select the one they desire.
I don't think this is an important update, but definitely a welcomed one.
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